answered the question by experiment. Two receivers were taken; one was filled with air compressed to about THE CHEMICAL NEWS. 20 atmospheres; the other receiver was exhausted by an VOL. XXXIX. No. 1005. ON THE LIQUEFACTION OF GASES.* By J. J. COLEMAN, F.I.C., F.C.S. THREE methods have been employed in the liquefaction of gases-I. Increase of pressure at normal temperatures. II. Abstraction of heat at normal pressures. III. The combination of these two operations. The gases which can be liquefied by pressure at normal temperatures are limited. Natterer showed us, some years ago, that under such conditions the enormous pressure of 3000 atmospheres was insufficient to liquefy hydrogen and oxygen. It was reserved for our countryman, Dr. Andrews, to demonstate the fallacy of relying upon pressure alone for gaseous liquefaction. To use the words of Prof. Wurtz in his recent Faraday Lecture, he has shown us that with all vapours there is a point at which the molecular movements caused by heat finally gain the victory over the force of cohesion, whatever be the pressure to which the air is subjected. Dr. Andrews describes it as the "critical point;" Mendelejeff names it the "absolute boiling-point." air-pump. The two receivers having been connected by a short pipe in which there was a stopcock, the entire apparatus was immersed in a canful of water. Then by opening the stopcock the compressed air was permitted to flow from the full to the empty receiver. The advocates of the old theory of heat would have said that since the compressed air in this operation dilated to twice its original volume heat must disappear to meet the increased capacity of air in consequence of its expansion. But the result, indeed, entirely contradicted this. There was no cooling effect whatever measurable by the most delicate thermometer, a result which certainly would have occurred had the compressed air been made to displace the atmosphere. In fact it has since been abundantly proved that the reduction of temperature which a compressed gas undergoes when allowed to expand into the atmosphere is owing to work done in displacing the atmosphere, which offers the well-known resistance of about 15 lbs. per sq. inch to the escape of the gas. Putting the facts into a brief statement, it may be taken as proved that when a compressed air or gas is allowed to expand the greater the resistance, and consequently the work done in overcoming such resistance, the greater the resultant cooling effect produced. Expand a compressed gas into a vacuum, no cold is produced; expand it in opposing the atmosphere, some cold is produced; expand it against a resistance such as that of a heavily loaded piston so as to contribute to its motion, and the maximum cooling effect is produced. Sir W. Thomson, Rankine, and Clausius have worked out the whole subject matheHe takes typical cases to illustrate the difference on the one hand of the expansion of a compressed gas as against the resistance of the atmosphere; on the other hand, as against the resistance of a loaded piston. Assuming an initial pressure of 5 atmospheres in each case, he shows that to bring the temperature of the expanded gas to the initial temperature before expansion, the first case would require 17 units of heat as against 74'9 required in the second case. It is, therefore, easy to understand why Natterer failed and Cailletet and Pictet succeeded with their experiments. Indeed, it would appear from these researches, and the principles underlying them, and if it be taken for granted that the molecules of all gases have a tendency to gravi-matically, but it may be convenient to quote Clausius. tate to each other independent of external pressure, that of the two agents for effecting the liquefaction of gases the abstraction of heat is more efficacious than the application of pressure. This is evident, and follows from the ordinary laws of gaseous expansion. It is a well-established fact that gases expand uniformly, and that the rate of expansion is such that a given volume of gas at the freezing-point of water will become double its bulk at 491° F. Therefore, if it be followed in its path of contraction for temperatures below freezing-point, by the time its temperature becomes reduced 491°, which is the absolute zero of physicists, the gas must necessarily be either liquefied or solidified. The greatest artificial cold yet produced has not exceeded about 220° below zero F.; but it would be rash to assume that it is impossible to produce greater cold. Indeed, it is not improbable that means may yet be found to produce cold sufficient to liquefy either oxygen or hydrogen at ordinary pressures. The question at once arises, what has already been done in the production of extremely low temperatures, and in what direction can such efforts be extended. But practice is dependent upon principles, and for principles we must fall back upon the mechanical theory of heat, thoroughly established and accepted universally by physicists, but as yet imperfectly understood by numbers of chemists educated in the old school of Black and Leslie. It is useful in the outset to recal certain experiments of Dr. Joule; and, parenthetically, it may be observed that it is to be regretted that no collected edition of the papers of this able man has yet been published. In his paper read to the Royal Society in June, 1844, some very interesting experiments with compressed air are detailed. The paper commences by reference to the well-known fact that compressed air when expanded becomes cooled, that is, when it escapes into the atmosphere; but Dr. Joule asked himself this question-" Would the cooling effect be greater or less were the same air expanded into a vacuum ?" He A paper read to the Chemical Section of the Philosophical Society of Glasgow and to the Institute of Engineers (Scotland). Full drawings of the machinery will be found in the Transactions of the latter body, published in Glasgow. Long before the mechanical theory of heat was enunciated Dr. Gorrie utilised for freezing-machines the intense cold produced by expanding a gas behind a working piston. Though his theoretical ideas were somewhat confused, Charles Randolph noticed and commented upon it in the case of the compressed air-engine erected at Govan Colliery in 1849. Air compressed to 20 to 30 lbs. to the square inch was sent down a shaft 176 yards deep, and along a road about 700 yards long, where it was used for working an old steam-engine in the place of steam, the result being that the ports of the engine cylinder were frequently blocked up with ice. The same phenomena can be observed in any kind of machinery worked with compressed air. In fact I have frequently observed the temperature of air issuing from the cylinder of coal- and rock-cutting machines to register fully many degrees below zero. Indeed, by such a process an unlimited reduction of temperature can be produced, provided the air or gas be subjected to sufficient compression before expansion, and supposing always that care be taken to remove the heat of compression by passing the compressed air through a surface condenser, kept cool with water, or by the actual injection of water at the time of its being compressed. When I first turned attention to the subject I exercised myself considerably in efforts to contrive a method of expanding air or gas so as to cause it to do the maximum of work in the act of expansion. But I speedily became convinced of the unlikelihood of anyone being able to invent a more perfect instrument for converting the expansion of an elastic fluid into work than is afforded by a well constructed steamengine. Mallard shows us that compressed air made to do work in an engine, at an initial pressure of 10 atmo spheres and temperature of 62°, should become reduced to -193° F. on being discharged, putting aside deductions for friction, &c. This illustrates the limitless production of cold by such a process; for let the initial pressure be higher, say 20 or 30 atmospheres, and the cooling effect will correspondingly be produced. Indeed, it seems to me that if Messrs. Pictet and Cailletet had in their experiments made the compressed oxygen or hydrogen do more work in expanding than the mere displacement of the atmosphere, then they would have been liquefied at much lower pressures. M. Pictet employed no less than 9000 lbs. pressure per square inch in his experiments. But it can be shown that a compression of only one-tenth of this, or 900 lbs. per square inch, raises a gas to 1200° F.; and that if gas so compressed be brought down to 60°, then expanded in the act of doing work, it must necessarily be cooled down to a lower degree than any yet experimentally shown, and which would probably be sufficient to effect the liquefaction of any gas we are acquainted with. and Again, in their experiments there must have been a production of heat militating against the liquefaction from the friction of the gases in escaping into the atmosphere through a minute orifice. This is the theoretical aspect of the case, but in practice several considerations arise. It is very difficult constructing the necessary apparatus strong enough without having such a mass of metals as to communicate heat rapidly to its contents; and, further, the friction of the piston rings and the rubbing surfaces of the valves, piston-rods, &c., is difficult to neutralise, lubrication at excessively low temperatures being troublesome. How far these difficulties may be overcome on original research remains to be proved; but they have been dealt with to some extent in machinery constructed under my direction for technical purposes, and it may be observed that this machinery differs essentially from apparatus previously used, inasmuch as its construction admits of the liquefied gases being produced in a continuous stream, whereas in all former experiments with the liquefaction of gases, a small volume has been first confined and liquefied, the experiment then coming to an abrupt end. In a paper read to the Chemical Society of London (Sept., 1875) I described the effect of pressure and cold upon the gaseous products of the distillation of carbonaceous shales, which remain permanent after all the condensable vapours within are removed in the usual way by cooling to atmospheric temperature. These gases, whilst being permanent at 50°, yield under the influence of a pressure of about 10 atmospheres and a cold of 200°, large quantities of volatile liquid hydrocarbons, consisting chiefly of butylen, amylen, hydride of amyl, and hexylen. The machine which is the subject of this paper was designed to deal with 300,000 cubic feet per day of this waste gas, by subjecting it to the cold and pressure just mentioned, and has now been working three years, having liquefied, in round numbers, about a quarter of a million gallons of these liquid hydrocarbons. It involves (1.) The pumping of the gas by steam-power into a system of tubes capable of being externally cooled, and from which condensed liquids can be drawn off by ball-cocks. (2.) Employing the compressed gas after being deprived of its liquid for working a second engine coupled with and parallel to the first, thus receiving a portion of the force originally employed in the compression. CHEMICAL NEWS, {February 28, 1879. Between these two motive cylinders is a heavy flly-wheel of about 7 tons weight. It became very evident in the early stages of construction that some kind of automatic regulator would be required, so that the speed of the machine should ne such as to exercise no undue suction upon the shale retorts, otherwise atmospheric air might have been taken into the machine in admixture with the hydrocarbon gases. This was accomplished by putting a small gasometer governor between the two motive cylinders just described, and adjusted so delicately that a variation of pressure equivalent to 0.25 would, by a system of levers, actuate a throttle-valve and open or close the admission of steam to the cylinder, and thus regulate the speed of the engine. These levers, when once adjusted, cause the engine to run at full speed of 80 revolutions when the pressure in the retorts rose to o'75 (or ) inch, and to run at half speed when the pressure sank to 0.25 inch. We will now trace the progress of the hydrocarbon gases. They are sucked into two pumps worked by the motive cylinders A and Ar, the piston rods of which extend to the two pumps, and are therein compressed to about 8 atmospheres absolute, or a little more than 100lbs. per square inch above the ordinary atmosphere. Under these circumstances some hundreds of degrees of heat are generated which require to be removed: this is accomplished in great part by injecting about 8 cubic inches of water into each pump every stroke of the engine. This water is forced into the gas-pump by small injection-pumps. The compressed gas is now conveyed by the pipes with injected water to the top of a surface condenser, and passes downwards through 200 iron tubes of about 1 inch diameter and 4 feet 6 inches long, these tubes being surrounded with cold water. The mixture of compressed gas, liquefied hydrocarbons, and injected water arrives at length at about 70° F. in reservoir at bottom of the surface condenser. The water and liquefied hydrocarbons are allowed to escape by an automatic regulated cock, and the compressed air is carried to the top of a second surface condenser, precisely similar in construction and with precisely similar outlets at the bottom reservoirs. It is then carried forward to the top of the third surface condenser, where it first passes downwards through a chamber packed with salt, so as to convert any suspended mixture into brine, and afterwards further downwards through 200 more 1-inch iron pipes in the case not surrounded with water. The compressed gas-still of about 8 atmospheres pressure is finally used for working the motive cylinder, which does not differ in construction from a steam cylinder, from which it is discharged after doing work considerably below zero Fahr. This constant stream of ejected and cold gas is made to circulate around the condensing pipes of the third surface condenser, and is finally carried away to be burnt as fuel under the boiler, being chiefly a residue of marsh-gas and hydrogen. The cycle is thus complete, and it only becomes necessary to indicate all the cylinders and note the volumes and temperatures of the incoming wtaer and gas and their final temperatures to have most interesting proof of some of the most important fundamental laws of thermo-dynamics. The machine was driven by steam of 50 lbs. initial pressure, and when it became desirable to indicate the cylinders the working pressure was kept as nearly as possible 100 lbs. to the square inch, and the piston speed moderate, circumstances it was found that the viz., 144 feet per minute (48 revolutions). Under these Total driving power.. .. 40 99 32 8 of The gas-pumps offered a resistance of.. Difference.. (3.) Employing the expanded gas after having had its temperature reduced in the act of doing the work of pumping for supplying the necessary cold for cooling a portion of the condenser pipes to zero. The machinery is kept in motion by the piston's cylin- The indicator cards show that the volume ders, A and A 1, the pistons of which are connected with compressed gas introduced into cylinder AI became rea common crank shaft, both being of 30-inch stroke.duced to atmospheric pressure by the end of the stroke. CHMMICAL NEWS, L Improved Method for Making Platinum-Alloy Assays. It is admitted to the cylinder not at the full pressure of 100 lbs., but by means of a reducing valve at about 60 lbs., being thus cut off at one fourth of the stroke. The recovered power seems comparatively small, but it must be taken into account that there is some loss of gas during its progress through the machine at the points where the liquid hydrocarbon are drawn off (probably 10 per cent); also, that there will be some of the gas liquefied, and that the residue of hydrogen and marsh-gas, being so very mobile, might have a tendency to pass between the piston block rings and the inside face of the cylinders. Taking these facts into consideration, the working of this cylinder has been very remarkable. It is generally coated externally with some inches thick of ice from the freezing mixture deposited upon it from the external atmosphere. The indicator cards of the gas-pump are very beautiful, and show a very near approach to the true isothermal curve of compression, resulting from the law of Mariotte. No doubt much of the beauty of this card is owing to the extremely small clearance spaces at the end of the stroke of the pump piston, viz., 0.25 of an inch, which again gets filled up with water from the injecting pumps. Such good curves also demonstrate the efficiency of the injection of the cold water by the small force-pumps, and the superfluity of elaborate arrangements, for throwing in pulverised water-insisted upon by Colladen, of Geneva, and other Continental engineers. This machinery has been frequently run at pressures approaching 150 lbs. to the square inch, and piston speed of 240 feet, without ever causing the pump-barrels or valves to reach a temperature of above blood-heat. 89 but there is no doubt even lower temperatures would have resulted if the capabilities of the machine had been stretched to their utmost limit. The hydrocarbon liquids obtained by this process are those produced by pressure alone escaping from the first and second, and those escaping from the third surface condenser at temperatures approaching zero, the result of combined cold and pressure. The former has generally a specific gravity of 0.710 at 60°, the latter of about 0.670 at 60°. The total produce of condensed hydrocarbons has been about 2000 gallons per week, which, when distilled, yield a considerable percentage of extremely volatile liquid hydrocarbons of specific gravities varying from 640° to o'660. These liquids, judging from the action of bromine, are chiefly amylen, hexylene, and dissolved butylen, with a small admixture of the corresponding hydrides, whereas the extremely light portion of American petroleum are in main composed of the hydrides of the alcohol radicles. The machinery I have just described was built for Young's Paraffin Light and Mineral Oil Co., for dealing with a portion of the gas produced at their works, probably about one-fifth. There are some who believe that the ends attained by this machine are as well accomplished by processes dependent upon gaseous absorption, the absorbent liquids being oils of heavy specific gravity; but my object in this paper is not to discuss this question, but to call attention to the process as the first application of Faraday's principle of the liquefaction of gases combined with an application of the principles of the mechanical production of cold, first foreshadowed by Count Rumford and Sir Humphry Davy, and developed by Joule, Clausius, Rankine, and Sir W. Thomson. ALLOY ASSAYS. The pump-valves are somewhat peculiar in construction, the arrangement being suggested by my friend, Mr. John Thompson, of Messrs. R. Laidlaw and Sons, the builders of the machine. Each pump end has four suction-valves of 1 ins. diameter, and three delivery-valves of 1 ins. IMPROVED METHOD FOR MAKING PLATINUMdiameter. Each of these valves work in a wrought-iron barrel, which contains the spring and the valve-spindle suitably guided. These wrought-iron barrels containing the valves are about 2 ins. diameter, and are externally grooved with a thread, so as to enable them to be screwed into the ends of the pump, or with the whole contents removed at pleasure. The piston blocks of the gas-pumps were originally provided with three-eighth square steel rings, but latterly it has been found that the common -inch square cast-iron ring wears best, and keeps the pumps in good working order for fully six months. The machine has been working continuously from Monday mornings to the following Saturday night as a rule without stopping, and has not been stopped for overhauling or repairs more frequently than the average of steam-engine machinery. Passing from mechanical details, as an example of results obtained by the machinery the following summary of consecutive observations made over a period of thirty days at the latter end of 1876 may be stated : Average temperature of the gas after being expanded and discharged from 30° below zero F. the motive cylinder A 1 Average temperature of the compressed gas on its way to be expanded 20° above zero F. after leaving the surface condenser S 3 Minimum temperature attained by the gas discharged from the motor 47° below zero F. cylinder HI.. Practical considerations as to wear and tear, steam consumption, &c., made it undesirable at that time to increase the working pressure above 135 lbs. to the square inch, By NELSON W. PERRY, E.M. ABOUT a year ago I had occasion to make some assays of platinum alloys. The first method employed consisted of cupellation with addition of Ag and Pb, the loss being base metal. The silver was obtained by precipitating from solution in HNO3 with HCl. This necessitated bringing the precipitate on a filter, washing thoroughly (giving a large filtrate for evaporation), drying, weighing, and calculating the Ag from weight of AgCl obtained. I will not detail this method further, but mention this last step to show one of the tedious steps in the operation. Doubtless many of your readers are familiar with this assay. The results by this method-strange to say, an established one-I found exceedingly unsatisfactory and inaccurate; one of the causes of its inaccuracy being that the percentage of platinum is obtained by difference, namely, all the other constituents being determined, their combined percentage is subtracted from 100. The error is therefore cumulative for platinum. The assay, or rather combined assay and analysis, as it is, required almost as much time as a good quantitative analysis, and yet could lay no claim to equal accuracy. I therefore set myself to work to devise, if possible, some better method, the results of which I set before you. In platinum alloys, or native platinum, the metals to be determined are base metal, Ag, Au, Pt, and iridosmine. Base metal is removed from the others by cupellation. Silver is soluble in H2SO4, while the others are untouched. Platinum when alloyed with twelve times its weight of silver, is soluble in HNO3. Gold is soluble in aqua regia, and iridosmine untouched by acids. Making use of the above properties, I was enabled to effect a separation of the metals both accurately and rapidly, no filtering being required, and all the washing being done by decantation. 90 Colouring Matters Derived from Diazo Compounds. Assay of platinum alloys containing base metal, silver, platinum, gold, and iridosmine. Charge Pt alloy 200 Mg, pure Ag 150 Mg, or sufficient Flatten button, anneal, roll out thin, anneal again, and make into cornet as in gold bullion assay. Introduce cornet into parting flask and part with concentrated H2SO4. Wash, anneal, and weigh. Loss from previous weight Ag in original alloy + Ag added for cupellation. Alloy cornet with at least twelve times the amount of Ag that there is Pt present, and, as before, form cornet and part first with HNO3 sp. gr. 116, and then HNO3 sp. gr. 126. Wash thoroughly, anneal in annealing cup, and weigh. Loss = Pt. Treat residue with aqua regia, obtain Au by loss-the residue is irridosmine. Time to complete assay in duplicate, 2 hours 45 minutes. The quality of Ag added should at least be sufficient, so that after the addition the Ag in the alloy will be to the Au as 3: I. Prerident-Warren de la Rue, F.R.S. Vice-Presidents-The list remains unaltered with the insertion of J. H. Gladstone and omission of Warren de la Rue. Treasurer-W. J. Russell. Secretaries-W. H. Perkin, H. E. Armstrong. Other Members of the Council-M. Carteighe, A. H. The PRESIDENT then called on Dr. OTTO N. WITT to read a paper "On Colouring Matters Derived from Diazo-compounds." During the last three or four years aniline colours have come into such general use as to, in many instances, replace the old costly artificial and natural dyes. Although there has been known for some time a great variety of magenta, violet, and blue aniline dyes, green and yellow dyes were almost unknown until quite recently. Three years ago the author, in a paper read before the Society, indicated a theory as to the relation between the chemical constitution and the colouring As platinum and irridosmine add greatly to the infusibility of the compound, silver in sufficient quantity must be added to prevent "freezing" and give a perfect cupella-power of aromatic substances. In this paper he pointed tion. Any large excess over these requirements is to be avoided-first, because the residue, after parting, will in that case be non-adherent and in a more or less fine state of subdivision, which may occasion loss in washing by decantation; second, the larger the button cupelled the more difficult it is to obtain a good cupellation, and the greater the loss of Ag during the process. It may, for this reason, sometimes be necessary to use only 100 Mg of the alloy for assay instead of 200 Mg, as above. The cupellation should take place at a moderate temperature, until near the "blick," when the assay should be thrust back into the hottest part of the furnace to prevent "freezing." The button must remain in the muffle until all the Pb is gone. In parting with H2SO4, boil for several minutes. In other respects, this operation is identical with the goldbullion assay. Any large excess of Ag over twelve times the amount of Pt in alloy is to be avoided, as it causes the residue, after parting, to be too fine and float, thereby occasioning loss in washing. Insufficient Ag is even worse, as the Pt will then be only incompletely dissolved. By the above method I was successful in obtaining very closely duplicating results, was enabled to do away with all precipitation and filtering, no apparatus being required, except that necessary for the gold and silver bullion assay, and, what is only second in importance to accuracy, attained results with the least expenditure of time. Mr. William Strieby, A.M., E.M., has assisted me materially in perfecting this assay, and employed it in his own work with equally satisfactory results. I present the above to your readers, hoping it may prove of service to some engaged in similar work.-Engineering and Mining Journal. PROCEEDINGS OF SOCIETIES. CHEMICAL SOCIETY. Dr. J. H.GLADSTONE, F.R.S., President, in the Chair. AFTER the confirmation of minutes, &c., the following certificates were read for the first time :-W. North, F. Podmore, W. Y. Gent, W. Radford. The list of Officers for the ensuing year as proposed by the Council was then read out that there existed a series of compounds, the colour of which was in close relation to their chemical constitution. The first link of this series is azo-benzene, C6H5-N-N-C6H5. This substance is of a deep yellow colour, but being devoid of salt forming groups is not applicable for dyeing. By introducing amido. or oxy. groups, compounds can be formed having strong affinities for textile fibres, which increase with the addition of each salt-forming group into the molecule. At the time this paper was read amido-azo-benzene and tri-amido-azobenzene were the only dyes of this class known; the first was of a beautiful bright yellow colour, but fugitive; the second was fast, but dull. Since that time the intermediate di-amido-azo-benzene has been formed and named chrysoidin, which combines the beauty of the monamido- and the fastness of the tri-amido-bodies. The preparation of chrysoidin is then given. Griess has shown that not only can amido-azo-compounds be obtained, but that the oxy-derivatives of azo-benzene and other similar bodies can be prepared by the action of diazo-compounds on the corresponding phenols; these oxyderivatives are also dyes; so that these di-azo-compounds have opened out an almost inexhaustible mine of new and beautiful dyes. Since Hofmann's paper on chrysoidin investigations on this class of compounds have been set on foot in the laboratories of almost all aniline colour manufacturers. The patents taken out for their production are constantly increasing in number. In consequence of his connection with Messrs. Williams, Thomas, and Dower the author has not been able to carry out his original intention of describing fully the preparation and properties of all his new azo-colours. The prototype of these compounds is chrysoidin. The author has already given a description of the properties of this substance in Fourn. Chem. Soc., ii., 457, 1877. He has since prepared in a pure state the following analogous substances closely resembling the typical product :—Ortho-tolyl-phenylenchrysoidin, para-tolyl-phenylen-chrysoidin, phenyl-toluylen-chrysoidin, ortho-toluylen-chrysoidin, para-tolyl-toluylen-chrysoidin, phenyl-toluylen-chrysoidin sulphonic acid, a-naphthyl-phenylen-chrysoidin sulphonic acid, a-naph thyl-toluylen-chrysoidin sulphonic acid. Each basic colour has an acid counterpart similar in shade and constitution but containing hydroxyl- in place of amido-groups. The acid counterpart of chrysoidin has been prepared by Baeyer and Fäger, and studied by Typke. This compound is a beautiful but unstable dye. The author therefore introduced a sulpho-group into its molecule by treating resorcin with para-di-azo-benzene sulphonic acid. substance has since been described by Griess. Its constitution that meta-di-azo-oxy-benzene sulphonic The 2. CHEMICAL NEWS, February 28, 1879) Action of Substances in the Nascent and Occluded Conditions acid. Its acid sodium salt is sold under the name of tropaolin O. By a similar reaction monoxy-azo-benzene sulphonic acid has been obtained from phenol. Its sodium salt is known as tropaolin Y. Tropæolin 000 No. I is oxy-a-naphthyl, and tropaolin 000 No. 2 the sodium salt of oxy-3-naphthyl-azo-phenyl sulphonic acid. Tropæolin 0000 is isomeric with tropaolin 000 Nos. I and Another class of azo-colours can be obtained if the salt-forming properties of amido-azo-benzene and analogous compounds be intensified by introducing one or more sulpho-groups. Thus, by acting with para-di-azo-benzene sulphonic acid on dimethyl-anilin, a dye is obtained, in which, however, the basic properties are too prominent, and to obtain a proper equilibrium one phenyl instead of two methyl groups must be introduced into the molecule of amido-azo-benzene sulphonic acid. Thus is produced one of the most beautiful of the azo-colouring matters. It is known as tropaolin 00. The author gives a detailed description of the preparation and purification of phenylamido-azo-benzene, which, when pure, forms leaflets or needles of a fine golden yellow colour; M.P. 82°; soluble in benzolin, alcohol, ether, and benzene. The author reserves for a future communication a description of amidodiphenyl-amin. Phenyl-amido-azo-benzene when treated with amylic nitrite and acetic acid yields a nitrosamin crystallising in orange needles, melting at 119.5°; its formula is C18H14N4O. By the action of di-azo-benzenesulphonic acid on diphenyl amin, tropaolin OO is obtained. It is a powerful acid and forms well-defined salts. The author gives a description of the potassium, sodium, ammonium, trimethylamin, barium, calcium, and aniline salts. In conclusion the author trusts that he has succeeded in giving a sketch of what may be called the genuine azo-colours, the true oxy- and amido-derivatives of azo-benzene, and analogous compounds. Compounds derived from amido-azo-bodies by the action of amines as well as coloured substances containing the azo-group, -N-N-, may also be termed azo-colours. The author hopes in a future paper to lay before the Society his researches on these more intricate compounds. The PRESIDENT said.that all must have appreciated the very lucid and brief manner in which Dr. Witt had described this beautiful series of compounds, interesting both from a scientific point of view and from their application as dyes. Mr. PERKIN said that the paper was of peculiar interest to him, as showing the enormous strides which aniline dyestuffs had taken during the last few years. He would like to ask whether these dyes were suitable for silk, wool, and cotton? Dr. ARMSTRONG remarked that the brief description given by Dr. Witt conveyed but a slight notion of the enormous amount of work concentrated in the results. He would ask Dr. Witt if he could point out the influence of the different groups in modifying colour? and also if he had any experience of tropæolin 00 as an indicator in alkalimetry? Dr. WITT said that most of the dyes, especially the sulpho-compounds, were only suitable for silk and wool, the amine compounds dyed cotton. He had already pointed out two groups concerned in the tinctorial power of a substance, which he had called chromogens and chromophors (Chem. Journ. Soc., ii., 403, 1876). Subsequent results had confirmed these conclusions. Oxy- and amido-g -groups principally had to do with the colour of the dyestuff. The sulpho-group weakened the colour but conferred stability on it. Also, cæteris paribus, the more oxyor amido-groups contained the more powerful is the dye. It does not follow, however, that the beauty of the colour increases with its intensity. The heavier the molecule the more the colour tends to the violet end of the spectrum. Some importance must be attached to the localisation of the salt-forming groups, i.e., the nearer they are together in the molecule the more powerful is the colour. Tropæolin 00 has been recommended as an indicator by Von Müller. It is not affected by CO2, and only after 91 some time by SH2, so that it can be used to titrate crude 48 Proportion of Reduction Grove's Time of I 2 I acid 68.2 per cent 3 hours 55 mins. 31'5 28.4 22 28.8 2 C.C. 19 8 24'2 2 4∞ 2 +8 4 2 water Each experiment went on till 35 cc. of oxygen collected at the anode. In the 2nd, 3rd, and 4th experiments of the series I to I the evolution of hydrogen at the cathode ceased quite suddenly at the end of three minutes. This is due to the presence of nitrous acid, which prevents the escape of hydrogen with eight cells when present in the proportion of o059 grm. to 100 cc. of 1 to 1 acid. Occluded Hydrogen and Nitric Acid :-Nothing is known as to the action of occluded hydrogen on nitric acid. Dr. Armstrong infers (Chem. Soc. Fourn., 1877, 82) that it has no action. The authors charged some finely-divided platinum with hydrogen. On pouring on it some pure nitric acid the metal became red-hot, the liquid became yellow, and nitrous fumes escaped; so that occluded as well as nascent hydrogen acts on nitric acid. Palladium charged with hydrogen dissolves in nitric acid 1 to I without setting free any gas, so that the hydrogen in this case must be oxidised. Nascent Hydrogen and Sulphuric Acid:-The acid was decomposed electrolytically. The authors conclude that hydrogen associated with platinum reduces oil of vitriol very readily, sulphurous acid being sometimes formed; a film of sulphur also appears on the negative electrode. Occluded Hydrogen and Sulphuric |