Page:Outlines of Physical Chemistry - 1899.djvu/59

 We shall follow the reverse course, and from the mechanical equivalent of heat deduce the difference which must exist between the specific heats of gases under constant volume and under constant pressure.

Let us consider a gram-molecule of any gas. If this be heated through one degree under constant pressure, it will expand by ^} % of its volume at 0°, that is, its volume increases

b * w - 81>98 c - c -

This expansion corresponds to a certain amount of external work : the atmospheric pressure (1,088 grams per centimetres.

This work is 84,685 gram-centimetres, and, since the mechanical equivalent of heat is 42,700 gram-centimetres per calorie, it is nearly equal to two small calories.

If the gas be heated under constant volume no external work would require to be done, and the quantity of heat which would be imparted to one gram-molecule of the gas would be less by two calories than in the previous case.

Hence, the molecular heat of a gas (specific heat x molecular weight) is greater by two calories under constant pressure than under constant volume. The following table gives a few molecular heats :

Under constant Under constant pressure rolume

2 6*96 4-96 calories

N 2 6-82 4-82 „

H 2 6*82 4-82 „

CO 6-8€^ 4-86 „

HOI 6-76 4-76 „

�� ��for chloroform 1*20, and for ethyl ether 1*029. It is evident that as the molecule becomes more complicated the ratio of the specifio heats diminishes.

The maximum ratio 1*666 is only realised in those cases where the molecule consists of a single atom, such as mercury and argon.

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