by 1 gramme of liquid Measures of vapour at 100° produced by 1 1633-1 488.3 1.700 0.661 0.402 0.411 39 7 35.4 2.7524 1.906 Bi 3:5756 2.34 DA 2.395 HD 2-424 G 3-1884 3.2088 G To 2.713 Το .... .... B C13 33.7 2.3364 182 1.2618 1.23 Da 1.2474 BA 1-2555 1-278 B G 1.2844 To 1.43 CO CI.... 49-4 3.4249 34604 To 3.6808 JD 58.5 4:0558 3.942 68.8 4.7699 4.875 Ds 69.47 4.8161 4.85 Mt 67.4 F? 18.7 1.2964 1-6841 FH? 9.85 0.6829 0.8871 Remarks on the preceding Table. AP. denotes Allen & Pepys; Be. Bérard; Bf. Buff; Bi. Bineau; Br. Brisson; Bt. Berthollet; Bz. Berzelius; BA. Biot & Arago; BD. Berzelius & Dulong; BG. Biot & Gay-Lussac; Ca. Cavendish; Cb. Couerbe; Co. Colin; Cr. Cruikshanks; CD. Clement & Desormes; Da. Dalton; Dm. Deiman; Ds. Dumas; Dz. Despretz; DB. Dumas & Boussingault; FS. Fourcroy, Vauquelin, & Seguin; G. Gay-Lussac; GT. Gay-Lussac & Thénard; H. Henry; HD. Humphry Davy; JD. John Davy; Jq. Jaquelain; K. Kirwan; L. Lavoisier; Ma. Marchand; Mt. Mitscherlich; Rg. Regnault; Ro. Rose; S. Saussure; Te. Thénard; Tf. Tromsdorff; To. Thomson; Tr. Tralles; W. Walter. The calculation of the specific gravity of gases rests upon the following suppositions: (1.) That the atomic weights of the elementary bodies are those given on page 50, and consequently, that if one measure of oxy gen gas weigh 16, one measure of nitrogen gas will weigh 14, and one measure of carbonic acid gas 22.-(2.) That the air is a mixture of 21 measures of oxygen, 78 95 of nitrogen, and 0.05 of carbonic acid gas. If x denote the sp. gr. of oxygen, y of nitrogen, aud z that of carbonic acid gas (that of air = 1) then 0-21 x + 0·7895 y + 0·0005 z = 1; moreover, 7x8y; and 11 x = 8 z. From this we find that the specific gravity of oxygen is 1.10926, that of nitrogen 0.9706, and that of carbonic acid gas 1-5252. From one of these three magnitudes, taken as a starting point, the specific gravities of the remaining substances have been calculated according to their atomic weights, and from the same data it has likewise been determined whether they undergo 0, 1, 3, 6, 9, 12, 18, or 24-fold expansion in the gaseous state. The specific gravities of those substances which in the table have a note of interrogation put after them, have never been determined by direct experiment; in the hypothetical calculation of them it has been assumed, according to analogy, that the vapours of selenium and tellurium are 6-atomic, vapour of antimony 2-atomic, and the vapours of carbon, boron, fluorine, hydrofluoric acid, silicium, titanium, bismuth, and tin, are 1-atomic gases. The differences between the results of calculation and experiment are easily explained in the case of vapours, because the exact determination of their specific gravity is subject to great difficulties. The differences between the results of calculation and observation in the case of the permanent gases perhaps arise from the adoption of incorrect hypotheses in the calculation, e. g. the atomic weight of nitrogen may have been assumed too low. Since, however, the results of the most distinguished observers frequently differ more from one another than from the calculated specific gravity, a new and exact revisiou of the specific gravities seems to be required, to enable us to decide positively respecting the correctness or incorrectness of the assumed hypo theses. One litre (or 1 cubic decimetre) of water at +4° (its point of greatest density) weighs 1000 grammes. 1 litre of atmospheric air under the 45th parallel of latitude, at 0° C. and 0.76 met. atmospheric pressure, weighs, according to Biot & Arago, 1.2991 gramme (according to Dumas & Boussingault 1.2995). This gives for the weight at 4°,-since at this temperature (p. 224) the air is expanded of its bulk at 0° (278: 274 1.2991: x), 1.28 gramme. To find the weight of a litre of any other gas at 0° and 0.76 met. bar. multiply the specific gravity of the gas by 1.2991 (the sp. gr. of air = 1). In this manner we may obtain with tolerable accuracy the relation by weight of water to the gas, since 1 litre of water weighs 1000 grammes; only it must be remembered that the weight of the gas is taken at 0°, and that of the water at +4°. If we would find the relation between the specific gravities of the gas and water at the same temperature, viz. +4°, we must assume that of water = 1000, and multiply the sp. gr. of the gas (that of air 1) by 1.28. = = According to the preceding, the specific gravity of air at 0°, and under a pressure of 0.76 metre, is to that of water at +4° as 1.2991 : 1000; hence (since 1.2991 : 1000 = 1:769-7) water is 770 times as heavy as air. If, then, the sp. gr. of air 1, that of water = 770; and if the specific gravity (that of water = 1) of any liquid or solid body be multiplied by 770, the product will be its specific gravity, taking that of air = 1. On the contrary, to reduce the sp. gr. of a gas (air = 1) to what it will be, taking water 1, we divide the former by 770. To find the expansion which a body undergoes in assuming the gaseous form, multiply its sp. gr. (water = 1) by 770 (the product will be its sp. gr. air = 1), = and divide this product by the sp. gr. (air 1) which it has in the gaseous state; e. g., in the case of water: 1.000 (sp. gr. of water) × 770 = 770; and 770: 0-6239 (sp. gr. of vapour of water) = 1234 1; for sulphur 2314; that is to say, 1 volume of water at 0° yields 1234.1 volumes of vapour of water at 0° and 0.76 met. pressure; and 1 volume of sulphur yields 231 4 volumes of sulphur vapour. 2.000.770= 66556 [Poggendorff's tables on the specific gravity of gases. (Pogg. 17, 259; 21, 629; 41, 449; 49, 416.)-Buff's method of determining the specific gravity of gases. (Pogg. 22, 242.) Dumas' Directions for determining the sp. gr. of gases. (Ann. Chim. Phys. 33, 341; also Pogg. 25, 236; also Ann. Pharm. 3, 59.) Mitscherlich's. (Pogg. 29, 193.)] C. Quantity of Heat in Gases. The quantity of combined heat in gases varies according to the nature of the ponderable body, and likewise according to the pressure to which the gas is subjected. a. According to the Nature of the Ponderable Body. The quantity of combined heat in the more permanent gases can only be approximately determined from the development of heat which takes place in the chemical combination of their ponderable elements with other substances to form non-gaseous compounds;-this development of heat is, however, partly due to the act of chemical combination. The quantity of heat set free in the absorption of acid gases and ammonia by water being but small, it would seem that these gases possess less heat of fluidity than vapour of water and some other vapours. The quantity of heat in vapours is found by conducting a known weight of the vapour (produced by boiling the non-gaseous element in a retort) through a worm-tube, or into a receiver surrounded with a known quantity of water-or, in the case of vapour of water, directly into the water-and observing the increase of temperature produced in the water by the condensation of the vapour. This increase of temperature (expressed in degrees) multiplied by the volume of water in the receiver (the weight of the condensed vapour being assumed =1) is equal to the heat of fluidity set free by the condensation of the gas, the decrease of temperature which the condensed matter undergoes from its boiling point to the temperature exhibited by the water at the end of the experiment.-Persoz (Chim. molec. 250) heats the liquid to its boiling point, and then drops it with a pipette upon mercury, the temperature of which is about 50° above the boiling point of the liquid, till the mercury is nearly cooled down to that point, and determines the quantity of heat which has become latent, by observing the temperature of the mercury before and after the experiment, the quantity of the mercury, and that of the liquid evaporated. Table of the Latent Heat of Vapours. Column A: Name of the substance.-B: Its specific gravity in the liquid or solid state.-C: Number of degrees centigrade above 0° which the vaporized body would exhibit at its boiling point, if none of the heat were rendered latent.-D: Deducting from this the heat required to raise the substance from 0° to its boiling point, the remainder is the quantity of latent heat in the vapour at the boiling point.-E: Number of degrees by which the temperature of water at 0° would be raised by the quantity of heat which the same weight of vaporized matter at its boiling point would give up, if it were to lose its gaseous condition and be cooled down to 0°.-F: The same after deducting the quantity of heat required to heat the substance from 0° to its boiling point, and found by multiplying the specific heat by the number of degrees at which the boiling point is fixed. This column F serves, therefore, for comparing the quantities of latent heat in different vaporized substances. If the numbers in columns E and F be divided by the specific heats of the respective substances, the quotients will give the numbers in columns C and D:--these numbers might be ob tained directly by experiment if the vapours of the several substances were passed, not through water, but through the bodies from which they are respectively formed, e. g. vapour of alcohol through cold alcohol, &c. -G: name of the observer. The quantities of heat required for vaporization appear to be to one another nearly in the inverse ratio of the densities of the vapours produced, and therefore also, in some measure, of the atomic weights of the corresponding substances: for on multiplying the latent heat of a vapour by its specific gravity, we obtain a set of numbers nearly equal to one another. If the latent heat belonging to the substance in its liquid state, which, however, is known only in the case of water, were added to the other portion of latent heat, a more exact coincidence would perhaps be obtained. This law, however, does not yet agree with the results of experiment sufficiently well to entitle it to be considered as established. (Comp. Th. Saussure, N. Gehl. 4, 97; also Gilb. 29, 126; Ure, Ph. Trans. 1818, p. 385; also Schw. 28, 360; Despretz, Ann. Chem. Phys. 24, 323.) T. I subjoin the following table of the latent heats of vapours recently determined by the experiments of Favre and Silbermann |