The ratios of the specific heats of several gases being found, it was highly expedient to find the ratio of the specific heat of water, and that of some one gas, as common air. This was effected by passing a small current of hot water through the calorimeter, and comparing the effect of this current with that of the larger one of air, the requisite care being taken to ascertain the quantity of water passing in a given time and its temperature at the ingress. The result of this experiment was that the specific heat of water is to that of common air as 1 ∶ .25 nearly. By two other experiments, varied from the above, results not much differing were obtained, so that the average of the three gave, water to air, as 1 ∶ .2669.
Reducing the specific heats of the gases to the standard of water as unity, we have the following Table of the specific heats of equal weights of the respective bodies:
| Water | 1.0000 |
| Air | 0.2669 |
| Hydrogen | 3.2936 |
| Carbonic Acid | 0.2210 |
| Oxygen | 0.2361 |
| Azote | 0.2754 |
| Nitrous Oxide | 0.2369 |
| Olefiant Gas | 0.4207 |
| Carbonic Oxide | 0.2884 |
| Aqueous Vapour | 0.8474 |
Before we animadvert upon these results, it will be expedient to give an abstract of the not less interesting experiments of Messrs. Dulong and Petit, on heat, as given in the Annales de Chimie and de Physique, vol. 7 and 10.
These gentlemen begin by an investigation of the expansion of air by heat. The absolute expansion of air from freezing of water to boiling had been previously determined by Gay Lussac and myself to be from 8 to 11 nearly: they however extended the enquiry above and below these points of temperature, namely to those of freezing and boiling mercury. From the temperature of freezing mercury or thereabouts, to that of boiling water, they find the expansion of air to keep pace with that of mercury, as indicated by the common thermometer; but from the boiling point of water to that of mercury, the latter expands somewhat more in a proportion gradually increasing: as by the following Table.
TABLE I.
| Temperature by Mercurial Thermometer. | Corresponding volume of a given mass of air. | Temperature by an air Thermometer, corrected for expansion of glass. | |
|---|---|---|---|
| Fahrenheit. | Centigrade. | Centigrade. | |
| -33° | -36° | 0.8650 | -36.8 |
| 32 | 0 | 1.0000 | 0 |
| 212 | 100 | 1.3750 | 100 |
| 302 | 150 | 1.5576 | 148.70 |
| 392 | 200 | 1.7389 | 197.05 |
| 482 | 250 | 1.9189 | 245.05 |
| 572 | 300 | 2.0976 | 292.70 |
| 680 | M. boil 360 | 2.3125 | 350.00 |
The absolute dilatation of mercury claims their attention. They quote nine authorities for the expansion from freezing to boiling water temperatures; the extremes of these nine are, Casbois ¹/₆₇ of original volume, and mine ¹/₅₀ of the same. They determine it to be ¹/₅₅.₅. By doubling and tripling the elevation of the temperature, they made observations from which are deduced the results of the following Table. The dilatations are for each degree of the thermometer centigrade, to which I have added the corresponding ones for Fahrenheit’s.
TABLE II.
| Temperature by an air Thermometer. | Mean absolute dilatations of mercury. | Temperatures indicated by dilatation of mercury, supposed uniform.[25] | |||
|---|---|---|---|---|---|
| Fahr. | Cent. | Fahr. | Cent. | Fahr. | Cent. |
| 32° | 0° | 0 | 0 | 32° | 0° |
| 212 | 100 | ¹/₉₉₉₀ | ¹/₅₅₅₀ | 212 | 100 |
| 392 | 200 | ¹/₉₉₄₅ | ¹/₅₅₂₅ | 400.3 | 204.61 |
| 572 | 300 | ¹/₉₅₄₀ | ¹/₅₃₀₀ | 597.5 | 314.15 |