ii. The temperature was indicated by the gas itself, and not by a

thermometer: iii. Water was easily separated previously from the gases, and excluded from the apparatus: iv. All the gases were placed in exactly the same circumstances, so as to render it unnecessary to refer to any calculation for the purpose of comparison.

Two methods of applying heat were resorted to: in one the balloon, containing the gas at a certain temperature, was placed in water at a higher but constant degree, for a certain time (generally 4′), and the elevation of temperature noticed: in the other, the balloon with the gas was inclosed in a larger copper balloon, blackened inside, and the space between the two exhausted as much as possible of air; the apparatus being then immersed in warm water, the heat gained access slowly to the gas, and the time of each experiment was increased, at the same time that certain sources of error were avoided.

The gases experimented with were, atmospheric air, oxygen, azote, hydrogen, carbonic acid, olefiant gas, oxide of carbon, nitrous oxide, nitrous gas, sulphuretted hydrogen, ammonia, sulphurous acid, muriatic acid, and cyanogen. Great care was taken in their preparation. The result of the experiment was very unexpected; for, during the five minutes allotted for each, all had acquired the same temperature,—a circumstance which proves that they all have the _same specific heat_. The equal volumes of gas at the pressure of 65 centimeters (15.59 inches) and the temperature of 20°C., being exposed to a source of heat at 30° C., acquired a mean temperature of 6.32 degrees in five minutes, the extreme difference, in any of the experiments, not being more than 0.04 of a [p201] degree. One gas only forms an exception to the above statement, namely, hydrogen, which was always heated more than the others, namely, to 6.6 degrees in the five minutes. This effect is considered as due not to any difference in specific heat, but to a difference in conducting power.

Experiments were then made with dilated gases, to ascertain whether dilatation caused any change in capacity, and it was found to diminish slowly but regularly with the diminution of pressure. These results, with a third which is also interesting, have been thus generally expressed by the authors at the end of their memoir.