59° POPULAR SCIENCE MONTHLY. 



be answered: Does a molecule of helium or of argon resemble a 

 molecule of oxygen in consisting of two atoms; or does it consist of 

 one atom or of more than two? 



It is believed that when heat is put into a gas, it is expended in 

 causing the molecules to move. This motion may in some cases be of 

 two kinds; the molecules may be urged through space, each molecule 

 traveling in a straight path, until a collision takes place with another 

 molecule, when it changes its rate and direction of motion ; such motion 

 i? termed 'translational motion.' On the other hand, if the molecules 

 are themselves complex, that is, if they consist of groups of atoms, any 

 energy imparted to the gas in the form of heat will produce, not merely 

 the translational motion, but will also cause the atoms to move rela- 

 tively to each other within the molecule. It is only the translational 

 motion which is manifested as pressure, for it is only by their motion 

 through space that the molecules can bombard the sides of the vessel 

 which contains them, and so exert pressure on the walls. Hence it 

 will require a less amount of heat to raise pressure in a gas with simple 

 molecules, than in one of which the molecules are complex, for in the 

 former case no heat is used in causing internal motion. Now, to 

 measure such quantities of heat is by no means easy, although it has 

 been successfully accomplished in some instances. To avoid this meas- 

 urement, a device is adopted which produces equally satisfactory re- 

 sults. It consists in comparing the amounts of heat required to raise 

 the temperature of a gas, first, when it is not allowed to expand, and when 

 all the heat is used in producing molecular motion of the kind referred 

 to; and second, when it is allowed to expand, and consequently when 

 it could be made to do work ; for example, to drive a small air-engine. 

 In the latter case, a greater amount of heat is required to rise the tem- 

 perature of the gas; an amount equivalent to the work which the gas 

 does on expanding. This quantity, however, which is equivalent to 

 mechanical work, is the same for all gases, provided equal numbers of 

 molecules (or equal volumes) be heated through the same number of 

 degrees of temperature. And this renders it possible to calculate the 

 amount of heat required to raise the temperature of a gas, even without 

 a direct measurement. An example will serve to render this somewhat 

 difficult conception clear. For mercury-gas, for argon and for helium, 

 and indeed for all gases, nitrogen, oxygen and their mixture, air, if a 

 volume which contains the molecular weight of the gas taken in grams 

 be raised through one degree of temperature, allowing the gas to ex- 

 pand, and so to do work, the amount of heat equivalent to this work 

 is sufficient to raise the temperature of two grams of water through 

 one degree. This is termed in the language of heat-measurement 2 

 calories. Now, the total heat required to raise the temperature of 40 

 grams of argon, for example (and it must be remembered that 40 is 



