I have said that if the flame of hydrogen becomes luminous under a high pressure, it arises from the temperature of the flame increasing in proportion as the pressure at which the combustion takes place itself increases. Let us now see what are the consequences of this fact, supposing it to be well established. M. Debray and I have proved that the temperature of combination of hydrogen and oxygen, under the ordinary pressure, is 2500°. We determined this fixed point by throwing into water a kilogramme of melted platinum raised to the highest temperature which could be produced in a lime-furnace, taking into account the increase in the temperature of the water, the specific heat of platinum and the law of its increase given by M. Pouillet, along with its latent heat as determined by M. Person. We should have liked to control so important a result by a great number of tests, and to fix it, as far as the data of calculation permitted, in an incontestable manner. For that purpose it would have been necessary to use large masses of platinum, and to protect ourselves against very serious accidents, terrible explosions, of which we narrowly escaped being the victims. We were closely occupied with the solution of this question when Professor Bunsen published his beautiful memoir on the temperature of combustion*. The excellence of the method invented by the great Heidelberg physicist made it unnecessary for us to recur to a tedious and dangerous method, the more so as the numbers obtained by Professor Bunsen are in the most complete agreement with our own. Professor Bunsen gives 2800° as the temperature of combination of the two gases purified and introduced in a state of absolute dryness into his valveeudiometer. Allowing for the moisture of the gases used in our experiments, and for the nitrogen brought into the gasholder by the water which displaced the gas, a number is obtained very near 2800°, which I shall adopt for the future as the true temperature corresponding to this phenomenon. Taking the number 2500°, I obtained the fraction 0.44† to represent the portion of the gases which really combine at the dissociation took place with the production of oxygen and of a yellow pulverulent and light carbon, to which, according to all appearance, is due the blue tint of the flame. M. Cailletet has observed that, in withdrawing and suddenly cooling the gases from the tuyère of a blast-furnace by means of my hot and cold tubes, these gases, produced by a carbon absolutely destitute of volatile matters, were rendered almost opaque by a sort of thick brownish fog, which after the lapse of some time was resolved into a blackish-yellow deposit of extremely finely divided carbon. * Pogg. Ann. vol. cxxxi. p. 161; Phil. Mag. S. 4. vol. xxxiv. p. 489. + Compare Leçons de la Société Chimique (de la Dissociation), p. 290 (Paris, Hachette, 1866). moment when (the heat of the mixture being a maximum) the dissociation of water corresponding to this temperature presents an obstacle to the complete union of its elements. Adopting the new number 2800°, we see that the part combined or not dissociated of the flame of hydrogen and of oxygen is really 0-50, or half the total mass. Bunsen's valve-eudiometer enabled him to investigate the temperature of combustion when the total pressure of the oxygen and hydrogen is diminished and is brought below the atmospheric pressure. It is sufficient for this purpose if a certain quantity of an inert gas be added to the explosive mixture. Under these circumstances Professor Bunsen observed that this temperature rapidly decreased in proportion as the partial tension of the explosive gases was made to decrease. Consequently the quantity of matter dissociated, or the tension of dissociation of water in the flame, decreases with the temperature. What would take place if we investigated the temperature of combination under a higher pressure than that of the atmosphere? This is obviously shown by Professor Frankland's experiments. To acquire absolute certainty on this point a striking verification is required, which may be obtained either by melting platinum in an artificially condensed atmosphere, or by repeating Bunsen's experiments with the valve-eudiometer. I am about to commence experiments of this kind; they will be made in a laboratory with iron walls capable of resisting a pressure of at least three atmospheres a pressure which the experiments made at the bridge of Kehl show is quite innocuous. It is easy to understand the practical consequences which may flow from a series of experiments made under pressure with the ordinary combustibles. They lead to a direct trial of furnaces fed with air forced under a pressure equal to the pressure of the vapour in the generator. These furnaces, especially if they are fed with the mineral oils, the use of which is already beginning to be recommended, and which leave no residue-these boilers where the products of combustion compressed to five atmospheres, for instance, would move through the tubes with one-fifth the velocity of our present apparatus, would doubtless enable the surface of heating to be considerably diminished. It is owing to the interest that investigations of this kind may have in furnishing naval engineers with the data necessary for calculating the results, that the Emperor has been good enough to order that these experiments be made in the laboratory of the Ecole Normale. A large cylindrical chamber capable of holding the operator and his apparatus, and of supporting a considerable pressure of air furnished by a steam-pump, will form a laboratory where all the manipulation necessary for determining the temperature produced by flames and solid combustibles may be effected without danger. If, as is almost already demonstrated by what I have said and by almost all the observations made by engineers and by physicists in chambers containing compressed air, the temperature of combustion rises at the same time as the pressure increases, that would be one analogy more to be added to the number of those I have indicated between the phenomena of combination and of decomposition on the one hand, and the phenomena of the condensation of vapours and of volatilization on the other hand. We may in fact give the name greatest temperature of condensation of vapour to what is improperly known as the boilingpoint of a liquid. This temperature is no other than that commencing from which a vapour no longer condenses on the surface of a cold thermometer, which is merely heated by means of the latent heat yielded to it by the vapour in which it is immersed. The boiling-point, or temperature of condensation, rises, as we know, when the pressure above the liquid which produces the vapour is increased. The combination of bodies, and particularly that of oxygen and hydrogen in the oxyhydrogen blowpipe, is apparently a more complex phenomenon, but corresponds perfectly to the act of the condensation of vapours. Assuming that the temperature of the combination of hydrogen and oxygen is 2800°, the quantity of water formed under a pressure of 760 millims. will be in the flame, at the hottest part*, 637+ (2800-100) 0.475 =0·5; 3833 that is to say, only half the oxygen and hydrogen will be combined under a pressure of 760 millims. But if we increase the pressure, the temperature of the flame increasing also, it will be seen from the preceding formula that the proportion of substance combined or of aqueous vapour formed will increase as the pressure increases-just as the tension of a saturated vapour increases in proportion as the temperature increases. Lastly, the temperature of combination of a gaseous mixture, like the greatest temperature of condensation (or boiling-point) of a vapour, increases with the pressure. The substance combined in a flame plays the same part as the substance condensed in a space full of vapour the temperature and pressure of which are varied so that the vapour is always saturated. * Vide Leçons de Chimie, given in 1864 and 1865, p. 290 (Hachette, 1866). It is clear from this that the quantity of substance uncombined or dissociated in the flame diminishes as the pressure increases. It may therefore be supposed that there is a pressure at which a mixture of hydrogen and oxygen would produce in combining the unimaginable temperature of 6800° which corresponds to total combination. But it is no more possible to make a serious hypothesis on this subject, than to ask whether there be a pressure at which water could no longer boil, whatever temperature were applied to it. I hope the Academy will excuse my having so long dwelt upon a mere programme of researches in course of execution; but they will be long and tedious, and I have been anxious to preserve the right of pursuing them if any one more fortunate than myself should sooner reach the object I am desirous of attaining. If the general considerations developed in this communication should facilitate the solution of a problem which I propound for the first time, and which I seek by paths which, if complicated, are yet rational, I shall be happy to have prepared the way. XVI. On Ethylate of Sodium and Ethylate of Potassium.-Part I. By J. ALFRED WANKLYN, Professor of Chemistry in the London Institution*. THE Ethylates of the Alkali-metals have been very imper fectly studied, and are well deserving of a minute investigation. Almost every one who has had occasion to prepare ethylate of sodium must have observed that the quantity of metal capable of being easily made to act on alcohol is comparatively small; from being very energetic, as it is at first, the action between the sodium and the alcohol soon becomes sluggish, and ceases long before so much as one equivalent of metal has decomposed one equivalent of alcohol. Nevertheless I believe that chemists usually regard the beautiful crystals which form when sodium is allowed to react upon alcohol as being ethylate of sodium, and as having the formula C2 H5 O Na. The crystals are in reality a compound of ethylate of sodium with alcohol. A note by A. Geuther and E. Scheitz shows that they consist of Na C2 H5 O, 2 (C2 H6O). (I quote from the Chemical News of January 8, 1869, which quotes the note on Ethylate of Sodium from the Jena Zeitschrift f. M. und N. vol. iv. p. 16). According to my own experiments, the crystals contain even more alcohol, viz. three molecules of alcohol to one molecule of ethylate of sodium, as will presently be described. It will also * Communicated by the Author. appear that the absolute ethylates of the alkali-metals are endowed with an extraordinary degree of stability, being among the most permanent compounds belonging to organic chemistry. Passing on to the description of my research :-A small glass retort, of 75 cubic centims. capacity (see figure), was cleaned, dried, and weighed. Into it was put some freshly cut sodium, the weight of which was ascertained. (Sodium admits of being accurately weighed, the thin film of oxide with which it so soon becomes covered being of insignificant weight.) Anhydrous alcohol was next poured into the Να retort, and the reaction between it and the sodium allowed to take place. The apparatus was then heated in the water-bath as long as any alcohol distilled over, and then cooled, dried, and weighed. The apparatus was a second time placed in the waterbath and subsequently cooled, dried, and weighed. The following are the numbers given by two experiments: 2.2 grm. 1.160 Weight of sodium employed and The crystals therefore consist of Na C2 H5 O, 3 (C2 H6 O), will bear a temperature of 100° C. without losing alcohol. They are in a state of complete fusion at 100° C., and so long as the air is excluded remain quite colourless. A very slight exposure to the air tinges them with brown-a remark which is applicable to the absolute ethylate about to be described. They are not very soluble in ether; in a mixture of acetic ether and ether they appear to be more soluble. Ön exposing them to temperatures above 100° C. these crystals give off alcohol, but they require a very considerable application of heat to drive off all the alcohol from them. |
