LORD KELVIN 301 



The temperature measured from absolute zero ( — 273°C) 

 is shown to be proportional to the kinetic energy of the gas 

 molecules; the pressure of a gas is the simple effect of the 

 collisions of the gas molecules with the walls of the vessel. 

 The tendency of every gas to spread through all space, al- 

 ready recognised by Guericke, is simply a consequence of the 

 straight line motion of the molecules according to Galileo's 

 inertia law, and is not caused by repulsive forces between the 

 molecules. The gas laws of Boyle and Mariotte, of Gay- 

 Lussac and Dalton, and of Avogadro, then follow of them- 

 selves as purely mechanical consequences of the molecular 

 motion, Avogadro's law was first put beyond all doubt as 

 a result of these connections, by its being linked with a large 

 number of well-established facts, both new and old. 



All this also led to Dalton's and Avogadro's views on the 

 atomic structure of matter, and the union of atoms to form 

 molecules, almost suddenly acquiring practically complete 

 certainty, which up to that time had been gradually ap- 

 proached throughout fifty years; and upon this basis a very 

 great deal more, now almost limitless in extent, was built up. 



Furthermore, it was not only possible to state the relative 

 weights of atoms and molecules, towards which science had 

 been feeling its way since Dalton, but to estimate their 

 diameters in ordinary units of length, and soon also their 

 absolute weights, and hence the number of them contained 

 in a cubic centimetre of every gas at given temperature and 

 pressure. The internal friction (viscosity) of gases, their 

 diffusion, and their heat conducting power, already studied 

 by Newton and Coulomb, could now be understood in every 

 detail and also quantitatively, with the result that their pecu- 

 liarities, in part quite unexpected in nature, could be pre- 

 dicted, and then confirmed by measurement. For example 

 gases, as opposed to liquids, increase in viscosity when 

 heated, that is to say, become less fluid. The result was 

 that quantitative relations were obtained, which could then 



