; ATOM. 



different substance* are proportional to the densities of these substances when in 

 the form of gas. The definition of the word molecule, however, as employed 

 in the statement of Gay-Luasac's law is by no means identical with the defi- 

 nition of the same word as in the kinetic theory of gases. The chemists 

 certain by experiment the ratios of the masses of the different substances in 

 a compound. From these they deduce the chemical equivalents of the different 

 substances, that of a particular substance, say hydrogen, being taken as unity. 

 The only evidence made use of is that furnished by chemical combinations. It 

 is also assumed, in order to account for the facts of combination, that the reason 

 why substances combine in definite ratios is that the molecules of the substances 

 are in the ratio of their chemical equivalents, and that what we call combination 

 is an action which takes place by a union of a molecule of one substance to 

 a molecule of the other. 



Tim kind of reasoning, when presented in a proper form and sustained by 

 proper evidence, has a high degree of ^cogency. But it is purely chemical 

 reasoning; it is not dynamical reasoning. It is founded on chemical experience, 

 not on the laws of motion. 



Our definition of a molecule is purely dynamical. A molecule is that 

 minute portion of a substance which moves about as a whole, so that its 

 parts, if it has any, do not part company during the motion of agitation of 

 the gas. The result of the kinetic theory, therefore, is to give us information 

 about the relative masses of molecules considered as moving bodies. The con- 

 sistency of this information with the deductions of chemists from the phenomena 

 of combination, greatly strengthens the evidence in favour of the actual existence 

 and motion of gaseous molecules. 



Another confirmation of the theory of molecules is derived from the experi- 

 ments of Dulong and Petit on the specific heat of gases, from which they 

 deduced the law which bears their name, and which asserts that the specific 

 heats of equal weights of gases are inversely as their combining weights, or, in 

 other words, that the capacities for heat of the chemical equivalents of different 

 gases are equal. We have seen that the temperature is determined by the 

 kinetic energy of agitation of each molecule. The molecule has also a certain 

 amount of energy of internal motion, whether of rotation or of vibration, but 

 the hypothesis of Clausius, that the mean value of the internal energy always 

 bears a proportion fixed for each gas to the energy of agitation, seems highly 

 probable and consistent with experiment The whole kinetic energy is there- 



