179.] EBULLITION-BOILING-POINTS. 377 (179) Ebullition.-The gradual absorption of heat in the passage from the liquid to the gaseous state is not less essential to the comfort, and even to the existence of man, than the corresponding absorption in the passage from the solid to the liquid Table of Boiling-Points and Densities of Liquids. Substances used. Sulphurous Anhydride Ethyl Chloride Aldehyd ... Methyl Formiate Ether Ethyl Bromide ... Carbonic Disulphide Formic Ether... Acetone Methyl Acetate ... ... -8·0* Silicic Chloride Bromine Pierre 63'0 145'4 3.1872 65'5 149'9 0'8179 Kopp ... 70'2 158 5 ... 73.8 16489 ... Benzol Trichloride of Phosphorus Dutch Liquid 1.6162 ... 80'4 176.8 ... 84.8 Methyl Butyrate Kopp Water Formic Acid Methyl Valerate Fusel Oil... Ethylene Dibromide Arsenions Chloride... Titanic Chloride ... Silicic Bromide 153'3 308'0 2.8128 Butyric Acid... 157'0 314'6 Sulphurous Ether 160°3 320'4 Phosphorous Bromide Sulphuric Acid 337-8 Mercury ... 350 0 640'0 1.8540 Marignac Regnault condition. Were it otherwise, every attempt to boil a saucepan or a flask of water or other liquid would be attended with explosion, from the sudden formation of vapour, the moment that the boiling-point was attained. * 10°5 C. Bunsen. + At -20°.5. 12067 at 13°5. At 20°.7. 378 MODE OF DETERMINING BOILING-POINTS. [179. By the term ebullition, or boiling, is meant the formation, in any liquid, of bubbles of vapour of a pressure equal to that of the superincumbent atmosphere at the time. Although the boiling-point of each liquid, cæteris paribus, is always fixed, yet different liquids vary quite as much in the temperature at which this change occurs, as solids do in their points of liquefaction. This is shown by a glance at the preceding table, which contains the boiling-points of a number of liquids, recently determined with very great care, reduced to the atmospheric pressure of 760mm. of mercury: the densities of the liquids at o° C. are also given. The process of ebullition may be beautifully shown in a common glass flask, heated from below. At first, bubbles of vapour are formed at the bottom of the vessel; these bubbles are condensed and disappear with a peculiar vibratory sound before they reach the surface; at length the temperature of the whole mass of liquid becomes nearly uniform, and the bubbles of steam as they are formed rise to the surface and break, emitting a perfectly transparent, invisible vapour, which does not become condensed into the cloudy form commonly but erroneously FIG, 136. designated as steam, until its temperature has been sufficiently reduced by the external air to bring it back to the liquid form in exceedingly minute globules. The temperature at which any given liquid boils, although perfectly fixed under certain conditions, is nevertheless influenced by several circumstances, such as-1, the nature of the vessel in which it is boiled; 2, the presence of matters in solution in the liquid; and 3, and most important of all, the variation of the pressure of the atmosphere upon its surface. The boiling-point of a liquid constitutes one of its most important physical characters, and is often the surest guarantee of its freedom from admixture with other bodies. The use of a simple expedient enables the boiling-point of a valuable liquid to be determined without loss, upon small quantities of the substance under trial. 180.] INFLUENCE OF ADHESION ON THE BOILING-POINT. 379 In fig. 136, F is a small flask which contains the liquid, t the thermometer passing through the cork c, and enclosed in a long tube e, which fits into a cork adjusted to the neck of the flask F: b is an outer tube to prevent the premature condensation of the vapour, d a lateral tube for carrying off any part of the liquid which may distil over; by this arrangement the bulb and the whole stem of the thermometer is immersed in the vapour of the boiling liquid, and an accurate observation may be made with little difficulty, due attention being paid in all cases to the barometric pressure at the time. (180) 1. Influence of Adhesion on the Boiling-Point.-Adhesion of the liquid to the surface of the vessel which contains it has a marked effect in raising the boiling-point. In consequence of this action, water sometimes boils at 101° C. in a glass vessel, but the temperature falls to 100°, and continues to boil steadily at this point, if a pinch of metallic filings be dropped in. If the interior of the vessel be varnished with shell-lac, the boiling will often not occur till a temperature of 105° C. is reached, and then will take place in bursts, the temperature falling to 100° at each gust of vapour. So again the presence of a little oil elevates the boiling-point of water 2° or 3° C. The experiments of Donny have thrown light upon some of the causes by which ebullition is facilitated. He has found that the presence of air in solution singularly assists the evolution of vapour. From the increased pressure which the dissolved air acquires by the addition of heat, minute bubbles are thrown off in the interior of the liquid, especially where it is in contact with a rough surface; and into these bubbles the steam dilates and rises. By long boiling of the water, the air becomes nearly all expelled; in such a case the temperature has been observed to rise even as high as 360° (182° C.) in an open glass vessel, which was then shattered with a loud report, by a sudden explosive burst of vapour. In such circumstances the cohesion retains the particles of the liquid throughout the mass in contact with each other, in a species of unstable equilibrium; and when this equilibrium is overturned at any one point the repulsion of the excess of heat stored up in the mass suddenly exerts itself, and the result is an explosion with the instantaneous dispersion of the liquid. The difficulty of expelling air completely, even from a small quantity of water, can be adequately conceived by those only who have attempted it; ebullition in vacuo for a very considerable period is not sufficient to effect it. In the slow freezing of water the air previously held in solution is perfectly expelled. In consequence of this absence 380 INFLUENCE OF SALTS ON THE BOILING-POINT. [180. of air, if a lump of ice free from air-bubbles be immersed in heated oil, so as to melt it without allowing it to come into contact with air, the temperature of the water may be raised many degrees above its boiling-point, and it will then be suddenly converted into steam with an explosion. Dufour finds that many liquids may be heated far beyond their normal boilingpoint by suspending them in the midst of a liquid of equal density, but which can be heated sufficiently without itself beginning to boil. If the globule of suspended and superheated liquid be touched with any solid body, it bursts into vapour with an explosion. Where the latent heat of the vapour is low, and the liquid has comparatively little adhesion to air, as is the case with alcohol, or ether, or sulphuric acid, frequent bumping or irregular boiling occurs, endangering the vessel and its contents. (181) 2. Influence of the Solution of Solids in a Liquid, on its Boiling-Point.-Any cause that acts in opposition to the repulsion of heat produces a corresponding rise in the boilingpoint; so that the solution of a salt in water, by the influence of adhesion, always elevates the point of ebullition, and the more so the larger the quantity of salt added. Indeed, it has been supposed that the quantity of salt required to produce a certain rise of temperature might be employed as a measure of the amount of adhesion between the liquid and the salt in solution. Legrand (Ann. Chim. Phys. 1835 [2], lix. 423) has published a series of careful experiments upon seventeen different salts, and the results which he has obtained possess considerable interest. It might be supposed, since the pressure of vapour increases with the temperature, that the addition of a larger quantity of salt would be required to raise the boiling-point from 101° to 102° than from 100° to 101° C. In only three cases, however, was this effect produced; these three salts stand first in the following table. In six instances the effect produced was exactly the reverse whilst in the seven instances which stand lowest in the table, the successive quantities of salt which it was requisite to add in order to produce a successive rise in the boiling-point of 1o decreased up to a certain point, and beyond this steadily increased. The salts employed were all used in the anhydrous state-that is to say, they were dried so as to be entirely deprived of their water of crystallization before being dissolved. Notwithstanding their high boiling-point, the vapour which rises from such solutions adjusts itself almost immediately to the 182.] INFLUENCE OF PRESSURE ON THE BOILING-POINT. 381 atmospheric pressure, and is not permanently hotter than the steam of boiling water, as Faraday and Magnus have shown.. In separating bodies by fractional distillation it is therefore necessary to maintain the bulb of the thermometer in the vapour, and not in the boiling liquid, since in the latter case the temperature indicated would give no information of the boiling-point of the liquid distilling. Influence of Salts in Solution on the Boiling-Point of Water. On comparing together solutions which contain equal quantities of different salts, it will be found that the most soluble salts are by no means uniformly those which produce the greatest elevation of the boiling-point. A solution containing 40 per cent. of common salt (very nearly saturated) boils at 226°4 (108° C.); whilst in the case of nitre (a far more soluble salt) a solution of the same strength boils at 219°·2 (104° C.). (182) 3. Influence of Pressure on the Boiling-Point.-Since ebullition consists essentially in the rapid formation of vapour of a pressure equal to that of the atmosphere which is exerting its pressure on the surface of the liquid, any diminution of that pressure should be attended with a corresponding depression of the boiling-point; and it is a fact that water which has long ceased to boil under the usual atmospheric pressure, may be at |