364 NINETEENTH CENTURY. PT. in. 



the densities of the gases, and we now know that the vibra- 

 tion of the molecules themselves is in this ratio. The 

 collisions also explain, as Clerk -Maxwell has shown, why 

 heat is conducted so slowly from one part to another of a 

 gas. The molecules which we make more energetic by heat 

 at one point hit against others and communicate their 

 energy, and these again to others, till they all move at 

 equal rates and the gas has an even temperature throughout. 

 And now, perhaps, we might think that, having arrived 

 at these infinitesimal movements among the molecules of 

 matter, we had gone as far as human power could penetrate. 

 But it is not so : Loschmidt in 1865, Stoney in 1868, and 

 Sir W. Thomson in 1870, were able, in following out the 

 discoveries of Clausius and Maxwell, to measure actually the 

 size of the molecule itself. They tejl us that two million 

 molecules of hydrogen placed in a row would measure one 

 millimetre, so that about 50 million would lie side by side 

 in an inch. Nor is this all, for the spectroscope has revealed 

 that even these minute particles are not mere rigid atoms. 

 We have seen (p. 337) that a gas gives a spectrum of bright 

 lines ; now these lines are caused by the vibration of the 

 parts of the molecule itself, in consequence of its having been 

 shaken by coming into collision with another molecule. Thus 

 we have to picture to ourselves these molecules not only 

 dashing to and fro and hitting each other, but each one of 

 them during its passage between one concussion and another, 

 quivering with the shocks it receives, and so giving out waves, 

 of coloured light, just as a bell when hit gives out waves of 

 sound The study of these vibrations and of the nature of 

 the atoms of which these molecules are composed, forms 

 some of the grandest work accomplished by Helmholtz and 

 Sir W. Thomson, while the theory of electro -magnetism 

 worked out by the late Professor Clerk-Maxwell rest"? upon 



