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deposit of arsenic, with its crystalline appearance and metallic brilliancy. There was thus deoxidation of part of the metalloid to form sulphate or arseniate of ammonia.

By immersing a crystal of sulphate of copper in a solution of polysulphide of potassium, the crystal soon becomes enveloped in a covering of sulphide of copper, on which most beautiful crystals of the rhomboidal form arrange themselves.

By immersing crystals of the protochloride of mercury HgCl, obtained by sublimation, in a solution of monosulphide of potassium, the chloride in a few days becomes transformed into crystallised cinnabar of a beautiful garnet red colour, and half the mercury is displaced. The instability of sub-sulphide of mercury HgCl explains this.

I have obtained gold in beautiful crystalline spangles by placing some chloride of gold contained in a porous vessel in the midst of a solution of protoxide of iron, of hyposulphite of soda, or of oxalic acid.

The reducing gases

tion on the cuprous solution. developed in the cistern had also acted.§§ I have also mentioned this deoxidising property of wood when in contact with sesquioxide of iron, in a work on the changes effected in the planking of ships by the nails and iron bolts. We also know that in stagnant waters, sulphate of iron is easily transformed into sulphide, and that crystalline clusters of pyrites often attach themselves to tufts of water reeds. There was an account in the German papers of a traveller who was lost in a mine, drowned in waters charged with sulphate of iron; after the lapse of some years the body was found coated with crystallised sulphide of iron, caused by the length of time it had lain in the ferruginous solution.

I placed a large crystal of sulphate of copper in a solution of monosulphide of potassium, and allowed all to stand for ten days. At the end of that time the outer surface of the crystal of sulphate was transformed into sulphide, and presented the exact external configuration of the primitive crystal.

I believe, then, that the formation of these crystals of copper must be attributed to a cause analogous to that which produces silvering and gilding by the wet way by means of reducing bodies; and finally to the reactions which organic matter exerts upon salts of copper and iron, and which cause the formation of native copper and iron pyrites.

The Analysis of Waters.

The explanation of the formation of sulphide of copper from this double sulphate is not difficult, but this cannot be said of the formation of metallic copper.

However, this unexpected production seems to me to be accounted for in the following manner:-We know how easily salts of copper are generally reduced, and that the prolonged contact of some organic matter is sufficient to bring about that reduction. Thus in ink, for example, there is often a deposit of microscopic crystals of copper, when sulphate of copper and a decoction of logwood were used in the manufacture. We also know that sugar effects this reduction with extreme facility; and, again, a stick of phosphorus left in a solution of sulphate of copper becomes, in a short time, enveloped in crystallised copper.

Lately, whilst going over the excellent mineral works of MM. Perret, of Lyons, I learnt a very interesting fact from those manufacturers-namely, that on emptying an old cistern which had for some time been filled with water charged with sulphates of copper and iron, moderately thick coverings of metallic copper were found attached to the wood which had served to support the roof of the cistern, and amongst the stones forming its floor. MM. Perret gave me some specimens they had collected, which presented clusters formed of a collection of octohedral and very large crystals. The débris of the wood had doubtless acted by reduc

of the various salts held in solution. A process which I have for some time worked gives most satisfactory results, saline matters. and shows beyond doubt the true composition of the

An ordinary potable water generally contains the chlorides of calcium, magnesium, potassium, and sodium (the chloride of calcium in extremely minute proportion). The sulphates of these alkaline earths and alkalies, with carbonate of iron, lime, magnesia, and silica.

One gallon or so of the water is brought to complete dryness at 212° F., lixiviated several times with boiling absolute alcohol, filtered, and washed with hot alcohol. This may be marked "alcoholic solution," and contains the chlorides only of calcium, magnesium, potassium, and sodium. This filtrate is boiled with water until all the spirit is driven off, and the chlorine and bases ascertained in three different portions. These results are calculated into chlorides; the chlorine serves as a check.

The residue insoluble in the alcohol is gently dried, boiled several times with water, filtered, the filtrate divided into three parts, and the lime, magnesia, potash, and soda, with the sulphuric acid, determined. Calculated into sulphates, the sulphuric acid as check.

The part insoluble in the water contains sulphate of lime (much of which has not been dissolved by the water), carbonate of lime, magnesia, peroxide of iron, and silica. It is dissolved in dilute hydrochloric acid,

§§ M. Clement Desormes had already proved that in the manufacture in the sides of wooden tubs. of crystallised sulphate of copper he could produce crystals of copper

the silica filtered off, the filtrate divided into two parts, and the sulphuric acid found in one, the iron, lime, and magnesia in the other. The sulphuric acid is calculated into sulphate of lime, and the excess of lime found into the carbonate; the iron and magnesia into carbonate. (The sulphate of lime found in the water solution is to be added to the sulphate got at this stage.)

The escape of chlorine from the chloride of magnesium on evaporation of the water to dryness I have ascertained to be most-minute, so that any objection on this score is removed, since it seems the large quantity of alkaline chlorides always present prevent this.

The total residue, organic matter, and silica should be determined in another portion of the water. In the evaporation of the water for analysis a large beaker or basin is preferred.

The advantages of this process are the solubility of the chlorides alone in alcohol, the solution of the sulphates in water, and the carbonates, &c., in the acid. R. C. M. Edinburgh, August 21.

PHARMACY, TOXICOLOGY, &c.

On Tobacco, by FERDINAND F. MAYER, of New York. (Continued from page 75.)

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after redissolving tested with iodohydrargyrate; this indicated 4'75 grains; only o'r per cent. were still present.

Commercial tobacco, known as Connecticut seed-leaf, and reported to be of the same variety and neighbourhood as the fresh plants under examination, presented the usual properties of good tobacco. Its simple watery infusion was decidedly alkaline. Hydrogen gas and still more air at slightly elevated temperature passed over the tobacco contained in a long tube, carried off very noticeable quantities of nicotina and ammonia. On distilling it with water, the vapours carried over nicoammonia, and its carbonate, and it was observed that the whole of the nicotina contained in a watery infusion could be evolved by boiling, or distilling it with a large excess of caustic ammonia.

As the result of several assays with two troy ounces at each time, I obtained as mean proportion of nicotina 1325 grains, equal to 136 per cent. of the leaf, precisely the percentage in the first assay as given above.

Tobacco seed of the same variety possesses scarcely any acridity, except what arises from rancidity of the oil. The latter is at times extracted as from other seeds, and employed for household use. Decoction with acidulated water was found to be the best mode for their extraction.

=

Of the uninjured leaves, with their petioles and ribs, 25 troy ounces were spread out to dry until their weight had been reduced to 1625 grains,-a loss of 86.5 per cent., though they were not yet perfectly air-dry.

Of this weight, only 245 grains had remained green throughout, or showed scarcely any decolorisation. The remainder had become more or less brown, on the upper surface principally, scarcely on the lower. They possessed a simple herbaceous odour, no pungency like that of ammonia, or commercial leaf, but a decidedly

acrid taste.

One hundred and seventy-one grains of the green leaves were exhausted with boiling alcohol-dilute alcohol acidulated with hydrochloric acid-and the alcohol removed from the tincture by distillation. The residue indicated, when tested with tenth normal solution of iodohydrargyrate of potassium (one cubic centimetre 00045 of a gramme = 0.0625 of a grain of nicotina), 2345 grains of the alkaloid in 171 grains. 136 per cent. of the dry leaf-01836 per cent. in the fresh.

Six hundred and sixty grains of the leaves which had become somewhat tainted were treated in precisely the same manner. They indicated within a mere fraction of 937 grains of alkaloid, 142 per cent. in the dry, 01917 per cent. in the fresh state. Of the fresh leaves, the juice of which was of a very decided acid reaction, 28.5 troy ounces had at the same time been placed in digestion at a moderate temperature for several days with water acidulated with sulphuric acid. The 3700 cc. of liquor resulting, contained, according to assay, 28.87 grains of nicotina, equal to 021 per cent. in the fresh state.

Ten troy ounces of the same were macerated in the same manner; the resulting liquor, however, evaporated in a porcelain dish to the consistence of soft extract, and

The leafstalks and ribs of the commercial leaf, and the liber of the green stalk, yielded noticeable quantities of the alkaloid, but their percentage was not determined.

For the purpose of identifying the alkaloid, or bases carried down by iodide of mercury from acidulated solutions in these several instances, the precipitates were washed with water-(since the test solution of iodohydrargyrate, besides containing a large excess of iodide of potassium, is too dilute to allow any trimethylina to be precipitated, no attention needed to be paid to any possible decomposition of the precipitate); it was then rubbed smooth in a mortar with a concentrated solution of protochloride of tin, to which were afterwards added small pieces of caustic potassa, so as to form an alkaline solution of protoxide of tin. The result in this, as well as in the case of all other bases, when their compounds with mercury are similarly treated, is that the mercury is reduced to the metallic state, its iodine or chlorine combining with the tin: the hydriodate of the base, then disengaged, is acted on by the excess of caustic alkali, and the base, when the mixture is now agitated with ether, is at once taken up in an almost pure condition.

After distilling off the ether, the residue is dissolved in dilute hydrochloric or oxalic acid, filtered, again shaken with alkali and ether or chloroform, which will then give a pure solution of the base, provided no lengthy evaporation has been made use of, in preparing liquors for precipitation with mercury.

The ethereal solution obtained in this manner from

precipitates of nicotina at first leaves the alkaloid as a pale, yellowish, oily liquid surrounded by a greenish brown soft resin or impure wax, which, next to ammonia, is the cause of the pungent odour of tobacco. To a certain extent it is a product of the oxidation of the alkaloid, but when repeatedly dissolved in alcohol it is odourless and tasteless, and contains no nitrogen.

Pure nicotina obtained from the second or third solu

tion in ether is colourless, has no odour at first, but acquires the pungency and odour of tobacco after some

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hours' exposure to the air, or on being heated. It agrees in most respects with the description given by Henry and Boutron-Charlard. Half a drop of it instantly killed a full-grown pigeon.

The liquids from which the precipitate of nicotina with mercury had been obtained, and which contained an excess of iodide, were, after filtering, rendered alkaline by the addition of caustic potassa. This causes a decided precipitate only in the case of commercial tobacco; the solutions from the dried leaves, in both the above experiments, gave a faint cloudiness. The filtrate from the seeds did not even show a trace of what would have indicated the presence of ammonia.

The precipitate produced in the first case was decomposed by alkali and protoxide of tin and then distilled; the distillate was converted into a platinum salt, which had all the properties of that of ammonia.

Having become satisfied, by my preliminary examination, that no stronger alkali than ammonia was required to expel nicotina as well as trimethylina, parts of the solutions to which no mercury had been added were severally distilled with caustic ammonia.

In the case of the acid infusion of fresh tobacco, no bases besides ammonia (from the retort) and nicotina were to be found in the gases and the liquids which came over and were received in hydrochloric acid. Evaporated to dryness, and tested for trimethylina, no traces of it could be discovered by the reactions described above, nor any particular resemblance in the odour; nor could any inflammable gas be extracted from the residual chlorides on heating with caustic potassa or baryta.

With an infusion of commercial tobacco, a great part of the nicotina comes over without the addition of an alkali, and nearly the whole quantity is obtained by adding a considerable excess of ammonia. This form is likewise free from trimethylina.

There is no perceptible difference in this respect between a decoction of the seed, and one of the fresh plant. The distillate contains only ammonia [intentionally] and nicotina.

From the watery distillate of ordinary tobacco, which had a milky appearance, but gave no deposit, and contained both ammonia and nicotina in solution, after acidulating with sulphuric acid, ether extracted a small quantity of a substance which I believe to be identical with the soft resin described before.

Though the experiments delineated so far are evidently yet incomplete, they still serve to answer the question proposed as completely as was possible at this season of the year.

The conclusions which I beg to submit are,1. That nicotina is the active principle in all parts of the plant before and after curing.

2. That there is in all probability no increase, but rather a loss of nicotina during the drying and curing, partly or wholly caused by volatilisation; and

3. That the plant or its parts contain no trimethylina, nor any ammonia while fresh,-Proc. Amer. Pharm. Asso., 1865.

Paris Exhibition of 1867.-A Report by M. Dumas to the Imperial Commission on the size and plan of the building necessary for the Exhibition is published in the Moniteur Scientifique. It states that at least 30,000 exhibitors may be expected to contribute, and space must be found for that number. The names of the French jurors in the division of agricultural products and manufactures have also been published, and we observe in one division or another the name of almost every chemist of note in Paris,

PROCEEDINGS OF SOCIETIES.

COLLEGE OF PHYSICIANS. "On Animal Chemistry." A course of Six Lectures by WILLIAM ODLING, M.B., F.R.S., F.R.C.P. Friday, May 5, 1865.

LECTURE 4.

Eternal relations of vegetable and animal processes-Nature of cosmical forces-Neutral convertibility of heat and motion-Their quantitative equivalency with one another -Illustrations of motion resulting from muscular effortForce rendered latent in mechanical separation of attracting bodies-Actual and potential energy-Chemical separation of attracting bodies-Theory of electrolysis-Force of galvanic battery derived from combustion of zincHeat of dissolving zinc manifested externally in ignition of platinum wire-The same heat stored up in electrolytic oxygen and hydrogen-Its reproduction by explosion of mixed gases-Solar heat rendered latent in separated oxygen and vegetable tissue-Its liberation by combustion of vegetable tissue in air-All terrestrial force traceable to the sun-Accumulation of solar force by vegetable organisms-Its dissipation by animal organisms-Reverse subsidiary actions-Baseless hypothesis of vital forceArtificial performance of alleged vital syntheses-Stages of constructive vegetal and destructive animal actionOccurrence of same intermediate products in both kingdoms- General processes of synthetic chemistry-Passage from one organic group to next in complexity-Continuous series of synthetic operations-Production of urea, formic acid, prussic acid, methylamine, and chloroform from mineral elements-Synthesis of di-carbon compounds, including alcohol, triethylamine, taurine, acetic acid, glycocine, and oxalic acid-Of tri-, tetra-, and penta-compounds, including glycerine, the lactic, butyric, succinic, malic, tartaric, and valeric acids, and fousel oil— Of hexacompounds, including caproic acid, leucine, and grape sugar (?)-Of hepta-compounds, including oil of bitter almonds, and the benzoic, salicic, and gallic acids-Possible artificial manufacture of food.

Ar the conclusion of my last lecture I was insisting upon and vegetable life in relation to the external forces of the the importance of considering the phenomena of animal universe. I observed that, however valuable might be the study of the more minute and elementary forms of life, and I was far from wishing to decry its value in any way, there were some great truths, on the other hand, which could only be appreciated by comparing the most highly differentiated forms of life with one another, not in their minute details, but in their broad general features. I observed, further, that while the chief cosmical function of highly developed vegetable life was deoxidation, or the separation of oxygen from carbon and hydrogen, the leading function of highly developed animal life was oxidacarbo-hydrogen with one another; and we agreed to intion-the recombination of the separated oxygen and vestigate upon this occasion the essential nature of these two correlative processes. I said, further, that I was induced to bring this subject under your notice in a more elementary and detailed manner than I originally contemplated, from finding that certain principles admitted by physicists to be as fundamental as the laws of gravitation were not heartily and unreservedly acknowledged by physicians.

First of all, then, we have to consider what is the character and import of that deoxidation of carbonic anhydride and water, which takes place in vegetable organisms; and by the exercise of what forces is it brought about. Now, in entering upon the discussion of this question, I must direct your attention for a short time to topics which at first sight seem but very remotely connected either with

chemistry or physiology. To paraphrase in sober earnest the expressions used in sarcasm by a very distinguished Fellow of this College, the value of whose contributions to physiological science no one can be more ready than myself to acknowledge, I shall preface my remarks by a few observations upon force and the constancy of its amount in the universe. Then, by a reference to systems and suns, and worlds, and steam-engines, and mills, and telegraphs, I shall endeavour to satisfy you that the same forces are at work in living plants and animals as in the inorganic world; and that the study of the indestructibility and correlation of force will teach us much, though very far from all, concerning the nature of life.

Let us begin, then, by considering the correlation that exists between the forces of heat and motion-their equivalency with, and convertibility into, one anotherof which no better illustration perhaps can be chosen than a steam-engine; and the philosophical toy upon the table known as Hero's clipile will answer our purpose perfectly well. On lighting a fire under the boiler of a steamengine, some of the heat liberated in the furnace gradually passes into the water; and as each successive increment of heat is transmitted from the furnace into the water, the temperature of the water rises until it arrives at a certain point. During the time the water takes in rising from its original temperature up to the boiling point, heat is being conveyed into it, at what may be regarded as an uniform rate. When, however, a certain degree of heat is reached, the temperature of the water no longer rises, although the heat of the furnace continues passing into it at the same rate as before; but now some of the heat absorbed, instead of being manifested in the form of heat, appears in the form of motion, and the steam-engine is set to work. Just as in the clipile before you, the heat of the spirit lamp was passing into the water for several minutes with no other effect than that of raising the temperature of the water; whereas, after the lapse of several minutes, the heat of the spirit lamp continually passing into the water ceased to increase the temperature of the water in the slightest degree, but caused, instead, a rapid rotatory motion of the instrument. Thus, we perceive generally that the development of motion is consequent upon an absorption of heat, and conversely, we shall find that an arrest of motion is tantamount to the liberation of heat. For example, if we employ our steam power in drilling a piece of metal, the motion of the engine is lessened by the friction of the drill, and a certain amount of heat is thereby generated. The original heat of the furnace, absorbed by the boiling water without any increase of temperature, produces a less amount of motion than formerly, but the diminution of motion is supplemented by an increase of heat-less of the heat which passes into the water being now manifested in the form of motion, but the difference reappearing at a distance in its primitive form of heat.

Similarly when a bullet strikes upon an iron target its motion is suddenly arrested, and its temperature as suddenly raised. That which was motion has become heat, and the quantity of heat shared between the bullet and the target is exactly proportional to the previous velocity of the bullet, or to the quantity of its motion that has been arrested. Again, in exact proportion to the diminution of motion in the working parts of the engine, so is the amount of heat developed by the friction of the drill upon the piece of metal. For not only are heat and motion convertible into one another in a general way, but they are convertible in a manner which admits of accurate measurement. By suitably contrived experiments it may be shown, in every instance, that so much heat developed is equivalent to so much motion arrested, and conversely that so much motion generated is equivalent to so much heat absorbed. It has been ascertained for instance, more particularly by Mr. Joule, that the force of a pound weight falling through 772 feet is exactly equal to the amount of heat which a pound of water will

give out in cooling one degree Fahrenheit; or, in other words, that the heat developed by the arrest of the motion of a pound weight falling through 772 feet would raise the temperature of a pound of water one degree, and that the heat given out by the cooling of a pound of water one degree would lift a pound weight through the height of 772 feet. Heat and motion, therefore, are in every case interchangeable for one another according to a definite standard of equivalency.

Now, let me direct your attention to some simple illustrations of motion; and, first of all, to the direct motion of a projectile hurled from the arm of a man. Upon seeing the rapid flight of a dart or javelin through the air, we recognise immediately that the motion of the javelin did not originate in itself, but was impressed upon it by some external force. We never think of attributing its motion to the exercise of any internal javelin force, but to the action of the muscular force by which it was originally projected. We know that the momentum of the javelin is exactly proportionate to the amount of force exerted in hurling it, and that when it strikes some distant object, the blow which that object receives is as surely struck by the man as if he had delivered it directly with his fist. Let us now take an illustration, in which motion is transmitted through the intervention of some simple instrument, as in the case of a bow and arrow, for example. Here, also, the motion of the arrow through the air does not take place in virtue of any force originating in the bow, but is consequent on the muscular force exerted in drawing the bow and cord asunder; and, limited only by the susceptibility of the bow, the momentum of the arrow depends entirely upon the amount of force exerted by the archer in pulling the bow-string.

Again, in discharging a cross-bow, the cord is first drawn over the catch by a muscular effort, and the arrow afterwards projected by the release of the cord. Although the susceptibility of this kind of bow is far more limited than that of a long bow, still it is evident that the force with which the arrow is eventually projected does not originate in the bow, but is only a transformation of the muscular force exerted in separating the cord and bow from one another. Now, suppose that, instead of pulling the trigger of the cross-bow immediately after the cord has been drawn over the catch, we allow the bow to remain in its stretched state for weeks, or months, or years, and then release the cord; still the force with which the arrow is eventually projected will be the original muscular force exerted at the moment of pulling the bow and cord asunder. Suppose, for instance, that I were to draw the cord of a cross-bow over the catch to-day, and a century hence some one were to release it by touching the trigger, still the force of the projected arrow would not be his force, but my force, exerted by me to-day, lodged in the bow for a hundred years, then manifested in the motion of the arrow, and finally transformed into heat upon the cessation of its flight.

Similarly the force with which a cannon-ball falling from the top of the Monument would strike the pavement below, would be the exact expression of the force exerted in lifting it to the top-that is to say, in separating the cannonball and the pavement a certain distance from each other, no matter how many years before. We may thus render muscular force latent in the stretched bowstring, raised cannon-ball, or other instrument, for any length of time. This latent force is generally spoken of as potential energy, while the active force exertable at any moment by the flying arrow or falling ball constitutes its actual or dynamic energy. Thus the actual energy of my arm becomes the potential energy of the crossbow, reappears as the actual energy of the projectile, and is finally not lost, but dissipated in the form of heat. The point, then, I wish particularly to impress upon you is that the actual mechanical energy manifested in the falling together, or springing together, of two separated bodies, the ball and the earth,

Let us now turn our attention to an altogether different kind of pulling apart, namely, the pulling apart of oxygen and hydrogen from their state of chemical combination. I am here repeating the well-known experiment of the electrolytic decomposition of water. By means of a galvanic battery I am tearing asunder the previously combined oxygen and hydrogen, and collecting the two gases in separate cylinders. Now, what is the nature of this separation, and how is it brought about? Seizing one link in the chain of actions as a starting-point, let us consider first of all the combination of zinc in the battery-cell; for in all ordinary batteries the direct or indirect oxidation of zinc is the source of the power obtained. Upon holding this bundle of loose zinc shavings in the blowpipe flame, you observe that the metal takes fire from time to time and burns with very great brilliancy, being converted into the white flocculi of oxide of zinc which are now floating about the room. If we introduce the ignited zine into a vessel of chlorine, it continues to burn, you perceive, with even greater intensity than before, producing abundant fumes of chloride of zinc. But we may obtain chloride of zinc more readily by dissolving the metal in hydrochloric acid, and in this case, also, the combination of zinc with chlorine is attended by an evolution of heat. Thus, the solution of granulated zinc in hydrochloric acid, now taking place in this gas generating flask, is being accompanied by a considerable elevation of temperature in the liquid, as well as by an evolution of hydrogen, according to the equation Zinc. Hydrochloric acid. Zine chloride. Hydrogen. Zn" 2HC1 + Zn"Cl2 + H2. Now, equivalent for equivalent, the quantity of heat liberated by the combination of zinc with oxygen or chlorine is much greater than that evolved by the similar combination of hydrogen; and, accordingly, when we burn zinc at the expense of hydrogen, as in this experiment, we obtain in the flask only so much of the heat produced by the burning of the zinc as is in excess of the heat absorbed in the unburning, so to speak, of the hydrogen. Hence, leaving out of consideration certain subsidiary phenomena, the heat produced by the solution of a given quantity of zinc in hydrochloric acid, and the heat producible by burning the thereby liberated hydrogen in an atmosphere of chlorine, added together, would exactly equal the amount of heat producible by burning the same quantity of zinc directly in chlorine gas, as we did a minute .or two ago. Thus, by the solution of zinc in hydrochloric or sulphuric acid, we have a certain proportion of the combination-heat of the zinc set free. The intensity of this heat is not great, in consequence of its being associated with so large a mass of matter in the flask, but its quantity is very considerable, and constitutes, indeed, the entire dynamic energy we have at our disposal in the galvanic battery.† When the zinc and platinum plates of the battery communicate freely with each other, the combination-heat of the attacked zinc is manifested solely by a rise of temperature in the contents of the cell, precisely as in the case we have just considered of the simple solution of granulated zinc in hydrochloric acid. But the battery is a machine for enabling us us to apply and transform this combination-heat of the

• Vide Professor Williamson's lecture "On the Dynamics of the Galvanic Battery," Phil. Mag., xxvi., 452.

Taking as our unit of heat the quantity of heat necessary to raise the temperature of a kilogramme of water 1° C.-that is to say, from o° C. to i C.-it is found that 65 grammes of zinc Zu", in com bining with twice 35'5 grammes of chlorine Cl2, to form 136 grammes of chloride of zinc Zn"Cl2, evolves 10131 units of heat; whereas 2 grammes of hydrogen H2, in combining with twice 35'5 grammes of chlorine Cl2, to form 73 grammes of hydrochloric acid 2HCl, evolves only 47 56 units of heat; hence in decomposing 73 grammes of hydrochloric acid by 65 grammes of zinc, according to the equation Zn'" + 2HCl = ZnCl2 + H2, we should have 10131 - 47'56=5375

units of heat liberated as initial battery power.

means of a platinum wire, you observe the small coil of wire assumes an intense state of ignition. Now, the heat of the platinum wire is nothing more than a portion of the translated heat of the zinc burning in the cell; which, instead of being manifested at the point of action in the cell, is manifested at a distance in the platinum wire; much as a portion of the furnace heat absorbed in the evaporation of water may be manifested at a distance by the friction of a drill. The heat exhibited in the platinum wire does not originate in the wire, but in the cell, and so much of the heat of the burning zinc as appears in the wire is lost to the cell. Just as the contracting muscle strikes its blow at a distance of many yards by means of the conducting javelin, so does the burning zinc strike its blow at the spot to which its impulse is conducted by the copper strands.

In electro-motive machines, the heat of the burning zinc is employed in doing mechanical work, just as the heat of burning coal is employed, though with far greater economy, in a steam-engine. But in the decomposition of water taking place upon the table, the heat of the burning zinc is employed in the chemical work of pulling oxygen and hydrogen apart from one another. Of the heat producible by the combustion of a given weight of zinc, the proportion manifested externally in the ignition of platinum wire, or employed externally in the separation of oxygen and hydrogen, is supplemental to the quantity absorbed in heating the cell; so that in the electrolysis of water, as in the heating of platinum wire, so much the more external work done, so much the less internal heat developed. By the electrolytic decomposition of water, the heat of the burning zinc, which does not appear internally in the cell, and which might be manifested externally in the ignition of platinum wire, lies dormant in the separated oxygen and hydrogen; but it is not lost. On the contrary, I can render it manifest to you with the greatest ease. Now that we have collected a sufficient quantity of our electrolytic gases, I have only to mix them together and explode them, when you observe a considerable evolution of light and heat resulting from their combination; which light and heat are nothing more than the light and heat of the burning zinc, not manifested in the cell, but retained for a time in the separated gases, so as to constitute their potential energy. The explosion of the gases at once or a hundred years hence would make no difference. The heat and light resulting from their eventual explosion would still be the heat and light of the burning zinc stored up in them, so to speak, at the moment of their electrolysis. In my last lecture I showed you the formation of water by the combustion of hydrogen in the oxygen of the air, and called your attention specially to the heat developed by the combination. The heat so developed did not originate in the oxygen and hydrogen, but was simply a liberation of the heat force, which, directly or indirectly, at some time or other, had been employed in pulling them apart, and been rendered latent in them so long as they continued apart-just as the force of a stretched crossbow does not originate in the bow, but is merely latent muscular force stored up in the separated bow and cord. Similarly in the rapid combustion of carbon in oxygen gas to form carbonic anhydride, there was no generation of heat, but only a liberation of the heat previously stored up in the two separated elements.

Now, then, we are in a position to understand the nature of the action taking place in the vegetable kingdom, by which carbonic anhydride and water are decomposed-to consider what is the external force employed in the pulling apart of oxygen from hydrogen and carbon, and what becomes of it. This force is no other than the light and heat force emanating from the sun, rendered of vegetable tissue or secretion on the other, and reprolatent in the oxygen on the one hand, and carbo-hydrogen