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hydrocarbons present. The explosive power of hydrogen, when mixed with its proper proportion of oxygen and fired, is about 26 atmospheres; of marsh gas, about 38 atmospheres; of olefiant gas, nearly 44 atmospheres; of propylene, about 66 atmospheres; and butylene, nearly 89 atmospheres; common coal gas, in exploding with its proper proportion of oxygen, exerts a pressure of about 34 atmospheres on the walls of the containing-chamber; and the proportion of air which is capable of giving the strongest explosion is 6 times the bulk of the gas.

In order that the leading properties of the several combustible constituents of coal gas may be seen at a glance, I will direct your attention to this diagram, where I have Constituents of Coal

tabulated not only the known constituents of gas, but also some of their allies, as the alcohol radicals, as they are sometimes called, which occupy an intermediate place between the marsh gas and the olefiant gas series of compounds. You will here see how in each series the proportions of carbon and hydrogen progressively increase in the same volume of gas or vapour, and how also the specific gravity of the gas likewise increases; and so also does its illuminating power, and its vitiating effect on the atmosphere. In this last respect I have considered that 4 per cent. of carbonic acid in the air renders it irrespirable. You will also note the proportions in which the several compounds are absorbed by water. Gas and their Allies.

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3. Impurities, or Objectionable Constituents of Coal Gas. These are Carbonic Acid, Oxygen, Nitrogen, Ammonia, Sulphide of Carbon, Sulpho-hydrocarbons, Sulphuretted Hydrogen, Cyanogen, and Aqueous Vapour.

(a) CARBONIC ACID (CO2).—This gas is always produced in the retorts during the early stages of the carbonisation of coal; and if the coals are very wet the quantity of the gas is increased by the action of the red-hot coke on the aqueous vapour-hydrogen and carbonic acid being produced. The amount of the gas is also augmented if the exhauster draws air through the pores or fissures of clay retorts. This gas consists of carbon and oxygen in such proportions that a volume of it contains half a volume of carbon vapour and a volume of oxygen. The gas is heavier than air in the proportion of 15202 to 1; and it is not only incombustible, but it also checks the combustion of other gases. A taper is immediately extinguished in air containing 14 per cent. of carbonic acid; and it burns very dimly, and only for a short time, in air containing 12 per cent. of the gas. Even detonating gas (a mixture of oxygen and hydrogen in proper proportions) will not explode when it is mixed with 2'89, or a little less than three times its volume, of carbonic acid. The effect of the gas in reducing the illuminating power of coal gas is very marked. You will observe this in the experiment which I will now make. Here is the standard flame of the burning gas, and when I introduce about 3 per cent. of carbonic acid into the gas, you will notice how seriously the power of the light and the volume of the flame are

2374 4 diminished. My own experiments have led me to the conclusion that I per cent. of carbonic acid in common coal gas will diminish the light about 5 per cent.; and that 2 per cent. of it in gas will reduce the power of 14-candle gas to about 12 5. In the case of the jet, a mixture of 5 per cent. carbonic acid reduces the light to half. It is, therefore, a serious impurity, and should always be removed from gas. This is easily accomplished, for carbonic acid is soluble at common temperatures, in its own volume of water, and alkalies absorb it very freely. Caustic lime, for example, will take up nearly its own weight of the gas; and, as I have already said, the purification should always be so managed that lime is the last agent through which the gas passes on its way to the holders.

Another objectionable property of carbonic acid is its action on the animal system. In its concentrated state the gas is absolutely irrespirable, and it kills immediately by causing spasm of the glottis. Even when it is diluted with air to the extent of 1 in 10, it quickly produces insensibility, and an atmosphere containing only 4 per cent. of it is dangerous. This is the proportion found in the air expired from the lungs; and therefore I have used it in the diagram just referred to as the proportion which renders air irrespirable. As little, indeed, as 2 per cent. of the gas in air will cause distress, as headache, and great depression of the vital powers; and no doubt the discomfort which arises from the too liberal use of gas in badlyventilated rooms is in great part due to the carbonic acid produced by the burning gas. I have found by experi ment that a small gas stove in one of Phillips's baths, in

my own bath-room, will charge the air so oppressively with carbonic acid as to render it almost irrespirable. On one occasion I detected 3.5 per cent. of carbonic acid in the air of the room; and I need not say that this is a dangerous quantity. It is therefore advisable that the products of the combustion of gas, when it is burned in large quantity, should be carried out of the room; and this precaution is applicable to every illuminating agent which vitiates the atmosphere, as you will see from this diagram:

Relative Values of Illuminating Agents,

In respect of their Heating and Vitiating Effects on the Atmosphere, when burning, so as to give the Light of 12 Standard Sperm Candles.

Cannel gas Common do. Sperm oil Benzole Paraffin

· Pounds of Water Oxygen Heated Consumed Cub. Ft.

1 Fahrenheit.

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6'25 156.2 8973 218.3 The mode of discovering the presence of carbonic acid in coal gas is very simple; for if the gas is passed through a little lime water it will render it milky, or if shaken with a solution of potash the amount of absorption indicates the proportion present.

(b) OXYGEN (0).-This comes from the too active operation of the exhauster, and the drawing in of air through the retorts. The mischievous influence of this gas on the illuminating power of coal gas will be seen from this experiment, where I will pass about 6 per cent. of air into the burning gas, and you will notice how seriously the light of the flame is destroyed. It has fallen, in fact, to about half its original power. The following are the proportions of light lost by different amounts of air in gas, supposing the light to be 100 :-

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The test for the presence of oxygen in coal gas is a little pyrogallic acid; and in operating you will proceed thus: First, agitate a known volume of the gas with a strong solution of potash, and observe the loss of volume-that is, due to carbonic acid; then put into the potash solution, which is still in contact with the gas, a little pyrogallic acid, and again agitate. If oxygen be present, the solution will acquire a brown colour, and the further diminution of volume is due to the absorption of oxygen. I ought to say that the gas is heavier than air in the proportion of 1 to 11056; and 100 volumes of water will absorb about 2.99 volumes of it at common temperatures.

(c) NITROGEN (N) is another impurity derived from the air by the action of the exhauster. The gas has no very marked influence on the luminosity of the flame, beyond this-that, in expanding under the influence of the temperature of the flame, it cools it somewhat by abstracting heat; and another objection to it is its power of forming nitric acid when it burns with the gas. There is no ready test for the discovery of this gas. It is a little lighter than air-its specific gravity being o'9713; and it is but feebly soluble in water-100 volumes taking up about 148 volume of the gas.

(d) AMMONIA (N H2).—This is a product of the carbonisation of the coal, and it may exist in the gas in a free

state, or combined with carbonic acid. A volume of the gas consists of half a volume of nitrogen and one and a half volume of hydrogen. Its specific gravity is o'5896, and its objectionable properties are that it attacks copper and brass fittings, and that in burning with the gas it forms nitric acid; and that it is a purveyor of naphthalin, and other heavy and fœtid hydrocarbons in coal gas. The gas is readily absorbed by water; at common temperatures water will take up about 727 times its volume of the gas. It is also absorbed by dilute acids, and by many neutral salts. There is, therefore, no difficulty in removing it from gas. The test for it is moistened turmeric paper, which becomes red in less than a minute when the gas contains no more than 1 grain of ammonia in 100 cubic feet.

(e) BISULPHIDE OF CARBON (CS,).-This compound is almost invariably present in coal gas, and it is no doubt formed in the latter stages of the distillation when the temperature is high, and when the sulphur liberated from the pyrites comes into contact with the incandescent coke. The vapour of bisulphide of carbon is very heavy-its specific gravity being about 2'6447; and its volatility is such that it cannot be condensed from gas by cold. It is not soluble in water, but it is freely dissolved by alcohol and ether, and by the volatile and fixed oils. When the vapour of sulphide of carbon is mixed with aqueous vapour, and passed through a red-hot tube, it is decomposed, and, by a mutual interchange of elements, the sulphur is converted into sulphuretted hydrogen, and the carbon into carbonic acid. It is also decomposed when it is passed over red-hot lime, or baryta, and when it is brought into contact with the peroxides of the metalsas of iron, manganese, and tin, in an ignited state; and, lastly, I may say that it is absorbed by sulphide of ammonium, and that it is decomposed by an alcoholic or aqueous solution of the alkalies in the presence of a salt of lead. All these reactions have been made the basis of inventions for the removal of bisulphide of carbon from coal gas. Thus the solution of it in oil is the property taken advantage of in the process of Schomberg, who proposes that the gas should be washed with oil. The decomposition of it at a red heat by steam is the recent suggestion of Mr. Lewis Thompson. The decomposition by ignited lime is the process of Mr. Bowditch; and its solution in ammoniacal liquor is the plan proposed by your president. The absorption of it by an alcoholic solution of potash or soda is the patent of Dr. Stenhouse; and the decomposition of it by passing it through a solution of oxide of lead in soda is the process of Dr. Angus Smith. And, although none of these processes have been found to be absolutely effective in practice, yet they are all more or less so; and they point to the importance of removing, as far as possible, this objectionable impurity from coal gas: for, in the act of burning, the sulphur of the bisulphide is converted into sulphurous acid, and this by further oxidation, in the presence of moisture, soon becomes sulphuric.

The tests for the discovery of this compound in coal gas are very numerous, and are founded on the reactions before mentioned. Dr. E. Herzog, for example, recommends that the gas should be passed through a strong solution of ammonia in alcohol, to which a few drops of sugar of lead has been added; and, if the bisulphide be present, it will give an orange-red precipitate. Or the gas may be passed into an alcoholic solution of potash, which freely absorbs the bisulphide, and converts it into xanthate of potash, which produces, with a solution of copper, the characteristic yellow precipitate of xanthate of copper; and, when boiled with a little sugar of lead, it produces a brown or black precipitate. Dr. Hofmann's test for bisulphide is to pass the gas through an ethereal solution of triethylphosphine, which forms with it a compound that crystallises in the form of little prisms of a splendid red colour. But the easiest mode of discovering its presence n coal gas, and also of determining its amount, is to burn

NEWS

the gas at the rate of 1 foot an hour from a Leslie burner, and to carry the products of combustion together with ammonia, into a glass cylinder, where they are condensed, and where the sulphur is afterwards found as sulphate of ammonia. The proportion of sulphur thus discoverable in coal gas ranges from about 4 grains per 100 cubic feet in the better kinds of cannel gas, to about 40 grains in the worse varieties of common London gas.

spiration collects as visible moisture. We, therefore, feel hot and uncomfortable. This is another reason why the products of the combustion of gas should be removed as speedily as possible from the room in which they are produced. And now I have but little time left for the discussion of the second part of my subject-namely, the means to be used for the combustion of gas so as to secure the fullest amount of heat or light. I will, however, direct your attention to two or three experiments, which will illustrate the fact that the light of a flame is dependent on the number of minute solid particles within it, and on the intensity and duration of their ignition. On the one hand, and which produces no solid particles in the act of burning, it will give me a flame of no appreciable luminosity; but I can at once give light to the flame by sifting solid matter, as magnesia or lime, into it; or by dosing it with benzole, or any other volatile hydrocarbon which is rich in carbon. Again, I have here the oxyhydrogen flame; it is barely perceptible until I direct it on a piece of lime, and then the solid particles of the lime become intensely ignited, and a vivid light is the result.

(f) SULPHO-HYDROCARBONS.-The nature of these compounds has not been determined, but there is good reason for concluding that sulphur exists in coal gas in other forms than as sulphuretted hydrogen and bisulphide of carbon. As in the last case, however, the sulphur, what-if I take hydrogen gas, which contains no solid particles, ever may be its form of combination, is discoverable by the combustion process.

(g) SULPHURETTED HYDROGEN (HS) is but rarely present in coal gas, and its proportion is never large. The test which is commonly used for its detection-namely, a little sugar of lead on paper-will discover the millionth part of this impurity.

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(h) CYANOGEN (C,N).-This compound is found in but very small proportion in coal-gas, although its presence is always indicated by the existence of sulpho-cyanogen in the water of the meters. It is a constant product of the carbonisation of coals, and its proportion increases with the amount of nitrogen in the coal, and with the temperature at which they are distilled. Nearly the whole of the cyanogen is condensed in the hydraulic main, where it unites with sulphuretted hydrogen and ammonia, and forms sulpho-cyanide of ammonium-the compound which gives the rich blood-red colour to the persalts of iron. But a trace of the gas escapes condensation, and passes into the mains, where it forms, in like manner, the sulphocyanide. In its pure state, cyanogen is a colourless gas, with a very peculiar odour, and it is a deadly poison. consists of a volume of nitrogen and a volume of carbon vapour condensed into one volume, and its specific gravity is 18006. It burns with a pale rose-coloured flame, and forms twice its own volume of carbonic acid. It is not easily absorbed by water, but it is more freely taken up by alcohol, ether, and volatile oils; and it is readily absorbed by alkalies and alkaline sulphides. It is this compound which, uniting with iron, forms Prussian blue, and so gives the blue and bluish-green colour to the spent lime (blue billy). There is no ready test for the recognition of the very small proportion of cyanogen contained in coal-gas; but if a large volume of gas be passed through a tube containing flints moistened with an alkali, and the flints be acidified, they will acquire a blue colour if iron be present, in consequence of the formation of Prussian blue; or the liquid may be treated with a little sulphate of iron, and then acidified with sulphuric acid, when it will present a blue colour.

(i) The last impurity in coal gas is AQUEOUS VAPOUR (HO), the proportion of which varies with the temperature. It is rarely less than 200 grains, or about 1037 cubic inches, in 100 cubic feet of gas, and it rarely exceeds 600 grains, or 3111 cubic inches, in the 100 feet. The influence of this impurity is not very striking, although the tendency of it is to reduce the luminosity of the flame by decomposing the hydrocarbons in the burning gas.

There is one property of aqueous vapour which has not been sufficiently noticed, but which is the chief cause of the heat and discomfort so often experienced in a room where gas is largely consumed. The property which I allude to is that of absorbing the radiant heat from the burning gas, and so, by its distribution through the air, elevating the temperature of the room. When much aqueous vapour exists in the atmosphere-and, as you will see by the table already referred to, there is much of it produced by burning gas-there is not only an elevation of temperature by the absorption of the radiant heat from the gas-flame, but there is also a check to the natural evaporation from the surface of our bodies, and the per

On the other hand, if I destroy the solid particles of a bright flame its luminosity disappears. Let me blow air into gas, as I can easily do with this double jet, and you the temperature of the flame is considerably increased, for will perceive how completely the light is destroyed; but I can now easily melt and burn iron with it. The difference, therefore, which we perceive in the light evolved by different flames is entirely dependent on the different proportions of solid particles contained in them. This spirit of wine and this sulphur burn with little or with a vivid light; and the results in the latter case are no light; but this phosphorus, and this magnesium burn due to the production of solid particles (phosphoric acid and magnesia), which are intensely heated. So again, the electric light is a stream of heated particles of carbon flowing from pole to pole. I have not time to enter on the question of the relative luminosity of these several flames, but you will which flows from it is, that when we wish to obtain the perceive they are all very great; and the consideration maximum amount of light from any quality of gas, the object should be to detain the particles of carbon in an ignited state as long as possible. This can only be accomplished by a proper adaptation of the supply of air; for if the supply be too great, the particles of carbon are too quickly burnt, and if it be too little, they escape unconsumed as smoke, and the temperature of the flame is reduced. You will see, therefore, that there is no one burner which is suited for every quality of gas, unless, indeed, the supply of it to the burner is regulated. I have here a number of contrivances which have been invented for the purpose of increasing the illuminating power of a poor gas; but they are all contrivances for checking the supply of of the gas. I regret we cannot enter further upon this air to the flame, or for diminishing the too rapid outflow subject, and that I can only show you by this diagram how much the form and measurement of the burner affect the quality of the light :

:

Illuminating Power of Common London Gas (13-candle), when burned from different Burners. Diameter Illuminating Prentage.

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All the 15-hole burners had a 7-inch chimney, and the
was burnt at the parliamentary rate of 5 cubic feet per
per hour; but the 30 hole burner (Bengel) had an 8-inch
chimney, and the gas was burnt at the rate of a little less
than 4
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calculated to feet. It is evident, therefore, that if a fixed
5
quantity of gas-say, 5 feet per hour-is to be passed
through the burner, the burner must be selected to give
the maximum amount of light, and therefore no fixed
measurement of burner can be specified; but if, on the
other hand, the burner is a fixed instrument, then the
quantity of gas passing it in a given time must be left
open, and the supply must be regulated to the best effect.
This, I believe, is the proper and the fairest means of esti-
mating the illuminating power of gas. It would then be
said that with a certain specified burner-as, for example,
the Bengel-a certain amount of light was given with not
more than a certain quantity of gas. This is the method
employed in France. In Erdmann's gas-prover, however,
the condition of things is reversed, as we are here estima-
ting the quantity of air necessary to destroy the light by
burning the solid carbonaceous particles, and therefore we
should experiment on the gas burning at a given rate-
say, o'84 cubic foot per hour. The instrument thus gives
reliable indications of the quality of the gas, as may be
seen from this diagram, which is taken from the experi-
ments of Mr. William King, of Liverpool.
Illuminating Power of Gas when burned at a given Rate
in Erdmann's Gas Prover—viz., at 0'84 Rate Cubic Foot
per Hour.

Height of flame (inch)
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ACADEMY OF SCIENCES.

June 26-continued.

M. PERSOZ contributed another extract from his important memoir on the molecular state of bodies. The extract related to the " Solubility of Salts," and contained a refutation of the laws deduced by Dalton from his experiments on solubilities. These laws, which are supported by recent experiments of Playfair and Joule, are stated as follows:

1. Anhydrous salts do not increase the volume of water in which they are dissolved.

2. Hydrated salts, in dissolving in water, increase its volume by an amount exactly equal to the water the salts contain. The above laws M. Persoz considered to be incompatible with the principle of the impenetrability of matter, and he undertook a series of experiments to elucidate the question. His method is not described in this extract, but a table of results is given, for which we have not space. We give, however, the conclusions arrived at by the experiments :

1. When an anhydrous or a hydrated salt, which has not the power of fixing water, is dissolved, the volume of the solution is equal to the sum of the volumes of the salt and water brought together.

2. The solution of an imperfect salt which fixes water is always attended with condensation, but this condensation never reaches the volume of the salt.

These laws, it will be seen, are quite opposed to those of Dalton. M. Persoz remarks that a simple relation always exists between the equivalent of a salt and the water which serves to dissolve it. The volume of a saturated solution of nitre at its boiling point (131° C.) is very nearly represented by two volumes of salt and one volume of water; that of a solution of plumbic nitrate at 100° C. by one equivalent of the salt and sixteen equivalents of

water.

M. Aristide Bérard sent a communication "On a New Direct Method of Producing Cast Steel by Means of Gases." (co-efficient0'7). 10°30 22.95 42.80 To get rid of the impurities of iron and produce his steel, Do. do. by photometer 10°30 23°58 42.96 the inventor of this process operates on the melted metal Relative values. 2.29 4'17 alternately with reducing and oxidising agents, in the The general laws which have been deduced from these shape of gases. Gases are also employed to generate the facts are,-1. That with all burners the maximum amount necessary heat. He operates in a sort of reverberatory of light, in the case of common gas, is always secured by furnace with two soles separated by a bridge. Coke is burning the gas at a pressure of from o'8 to o'12 of an placed on this bridge to remove free oxygen. While air inch of water at the orifices of ignition. 2. That with is sent through the metal on one side, the other is treated Argand burners of 15 holes the best size is the o'06 of an with a mixture of hydrogen and carbonic oxide free from inch for each hole, and with 30 holes the measurement sulphur. After fifteen minutes the treatment is reversed. should be the o'024 of an inch; and with fish-tails or The process is explained thus:-During the period of union burners, the holes should be the o'06 of an inch, and oxidation part of the iron is oxidised to protoxide; the the slit of the bat's-wing the o'03 of an inch wide. 3. earthy metals, such as silicium, aluminium, calcium, &c., That the consumption of gas should be regulated in the are also oxidised and probably combine with the oxide of Argand to a little below the top of the chimney, and in iron and form compound silicates. The sulphur, phosthe fish-tail and bat's-wing it should not range beyond phorus, and arsenic also oxidise and pass away. In the from 3 to 4 feet per hour. 4. As the measurement of period of reduction, the oxide of iron is reduced and the the burner has much to do with the supply of air to the other metals remain in combination with the silica forming gas, the external diameter of an Argand burner with 15 scoria, which swims on the surface. Any sulphur, phosholes and a 7-inch chimney should be 1'1 inch, and the phorus, or arsenic which escaped oxidation will now form internal diameter 0'44 inch; while in the 30-hole burner a volatile hydrogen compound. The final process is that and 8-inch chimney (Bengel), the external diameter should of decarburation, effected, we presume, by the air-blast. be o 89 of an inch, and internal diameter o 35 of an inch; In this way the author states that he gets cast steel of firstand the diameter of the nipple of the fish-tail or bat's-rate quality. Manganese, he says, plays an indefinite part, wing should range from o'22 of an inch to o'35.

These are the leading facts to be kept in view; and now, in conclusion, I have to say that there yet remain many important points for consideration, not only as regards the combustion of gas for economic purposes, but also as regards the chemistry of the waste products, as they are called in the manufacture of gas; and these I hope to have an opportunity of elucidating.

Dr. Letheby was ably assisted in his lecture by Mr. Sugg, who provided all the instruments and burners used during the lecture.

the certain effect of which is to facilitate the conversion of the iron into steel.

M. Marignac, in a note "On the Constitution of Hyponiobic Acid, and Tantalic and their Association in Minerals," states that he has experimentally demonstrated the truth of an hypothesis he published some months ago, that hyponiobic acid has the formula Nb,O,. He infers this formula from the constitution of the fluoride. He recommends, however, that the name should be changed to niobic acid, the name first given by Rose. The pelopic acid of the same author M. Marignac would designate niobous acid.

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N6 Benzoic aldehyde at 120° gives a crystalline product, having very similar properties to the foregoing. The author observes that the substitution products furnished by aldehydes with rosaniline support Dr. Hofmann's views of the constitution of aniline red and blue.

M. R. de Luna sent a note "On the Deposits of Phosphate of Lime in Estremadura." It contained some rough analyses, from which we quote the amounts of tribasic phosphate found in several specimens. Caceres: Maximum, 72°10; minimum, 50°10. Montanchez, 85'03. The mines from which these specimens have been taken do not belong to the English Company; and the author states that he has felt it his duty to call the attention of the Spanish Government to the misfortunes that will result if these deposits are allowed to fall into the hands of Englishmen.

On Radiation.

NOTICES OF BOOKS.

| Falling on white paper, the image clears itself out; falling on black paper two holes are pierced in it, corresponding to the images of the two coal points; but falling on a thin plate of carbon in vacuo, or upon a thin sheet of platinised platinum, either in vacuo or in air, radiant heat vivid incandescence upon both the carbon and the metal. is converted into light, and the image stamps itself inResults similar to those obtained with the electric light have also been obtained with the invisible rays of the lime light and of the sun.

The "Rede" lecture delivered in the Senate-House before the University of Cambridge on Tuesday, May 16, 1865. By JOHN TYNDALL, F.R.S., &c., &c. London: Longman and Co. 1865. A SHORT time ago the University of Cambridge did itself an honour by conferring an honorary degree on Dr. Tyndall; and in return, if we may so say, Dr. Tyndall delivered before the University this eloquent lecture, and illustrated it in his usual brilliant and happy style. The subject is peculiarly the doctor's own, and we need hardly say that the discourse is principally occupied with those important researches the results of which have been communicated by the author to the Royal Society within the last year or two. These results have been laid before our readers as they were published, but we would, nevertheless, recommend every one to re-peruse them as brought together in these eloquent pages. On looking for an extract, we find none that will be more instructive to many of our readers than the short section on the transmutation of rays, the conversion of heat into light :

"Eminent experimenters were long occupied in demonstrating the substantial identity of light and radiant heat, and we have now the means of offering a new and striking proof of this identity. A concave mirror produces beyond the object which it reflects an inverted and magnified image of the object; withdrawing, for example, our iodine solution, an intensely luminous inverted image of the carbon points of the electric light is formed at the focus of the mirror employed in the foregoing experiments. When the solution is interposed and the light is cut away, what becomes of the image? It disappears from sight, but an invisible thermograph remains, and it is only the peculiar constitution of our eyes that disqualifies us from seeing the picture formed by the calorific rays.

"Before a Cambridge audience it is hardly necessary to refer to the excellent researches of Professor Stokes at the opposite end of the spectrum. The above results fessor Stokes named the phenomena which he has disconstitute a kind of complement to his discoveries. Procovered and investigated fluorescence; for the new phenomena here described I have proposed the term calorescence. He by the interposition of a proper medium so lowered the refrangibility of the ultra-violet rays of the spectrum as to render them visible; and here by the interposition of the platinum foil the refrangibility of the ultra-red rays is so exalted as to render them visible. Looking through a prism at the incandescent image of the carbon points, the light of the image is decomposed, and a complete spectrum obtained. The invisible rays of the electric light, remoulded by the atoms of the platinum, shine thus visibly forth, ultra-red rays being converted into red, orange, yellow, green, blue, indigo, and ultraviolet ones. Could we, moreover, raise the original source of rays to a sufficiently high temperature, we might not only obtain from the dark rays of such a source a single incandescent image, but from the dark rays of this image we might obtain a second one, from the dark rays of the second a third, and so on-a series of complete images and spectra being thus extracted from the invisible emission of the primitive source.'

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We need only add that the lecture is published at a price that makes it accessible to every one, and, giving as it does a glowing panorama of the whole subject of radiation, is certain to be eagerly read by all who wish to be informed of the facts of these, perhaps, most important of natural phenomena.

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Poggendorf's Annalen der Physik und Chemie. No. 5. 1865. The number opens with the second part of a very valuable paper by Knoblauch, "On the Diffusion of Heat Rays." This is followed by a contribution to Knowledge of Nitride of Iron," by Dr. Stahlschmidt. The author first notices the researches of Fremy on the origin and constitution of steel, and of Berthollet, Thénard, Savart, and others on nitride of iron. then describes the method by which he obtained the nitride. This was the process of Regnault and Fremy, who pass dry ammoniacal gas over ferrous chloride, which greedily absorbs ammonia, forming ammonio-chloride of iron. When the tube containing this is heated, the mass fuses and becomes of a dark colour, ammonia being evolved. A pale yellow, porous mass remains behind, from which, on the application of a stronger heat, nitride of iron and sal-ammoniac are formed. Simultaneously a large quantity of hydrogen escapes in consequence of an evident decomposition. The composition of the nitride of iron varies somewhat with the temperature at which it is formed, and in general the higher the temperature the smaller is the proportion of nitrogen it contains. obtain a compound of constant composition, the ferrous chloride must be presented to the ammonia in very thin layers, and the heat applied be but just sufficient to volatilise the sal-ammoniac. The process is ended when no more vapour of sal-ammoniac is evolved; and then the passage of ammonia is continued a short time, and the tube is allowed to cool filled with the gas. In this way nitride of iron is obtained partly in thin scales, and partly

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