4 On Radiant Matter. 



If, in the beginning of this century, we had asked, What is 

 a Gas ? the answer then would have been that it is matter, 

 expanded and rarefied to such an extent as to be impalpable, 

 save when set in violent motion ; invisible, incapable of as- 

 suming or of being reduced into any definite form like solids, 

 or of forming drops like liquids ; always ready to expand 

 where no resistance is offered, and to contract on being sub- 

 jected to pressure. Sixty years ago such were the chief 

 attributes assigned to gases. Modern research, however, has 

 greatly enlarged and modified our views on the constitution 

 of these elastic fluids. Gases are now considered to be com- 

 posed of an almost infinite number of small particles or 

 molecules, which are constantly moving in every direc- 

 tion with velocities of all conceivable magnitudes. As 

 these molecules are exceedingly numerous, it follows that 

 no molecule can move far in any direction without coming 

 in contact with some other molecule. But if we exhaust the 

 airorgas contained in a closed vessel, the number of molecules 

 becomes diminished, and the distance through which any 

 one of them can move without coming in contact with 

 another is increased, the length of the mean free path being 

 inversely proportional to the number of molecules present. 

 The further this process is carried the longer becomes the 

 average distance a molecule can travel before entering 



belief in the association of the radiant form with the others in the set 

 of changes I have mentioned. 



" As we ascend from the solid to the fluid and gaseous states, phy- 

 sical properties diminish in number and variety, each state losing some 

 of those which belonged to the preceding state. When solids are con- 

 verted into fluids, all the varieties of hardness and softness are neces- 

 sarily lost. Crystalline and other shapes are destroyed. Opacity and 

 colour frequently give way to a colourless transparency, and a general 

 mobility of particles is conferred. 



" Passing onward to the gaseous state, still more of the evident cha- 

 racters of bodies are annihilated. The immense differences in their 

 weight almost disappear ; the remains of difference in colour that were 

 left, are lost. Transparency becomes universal, and they are all elastic. 

 They now form but one set of substances, and the varieties of density, 

 hardness, opacity, colour, elasticity and form, which render the num- 

 ber of solids and fluids almost infinite, are now supplied by a few slight 

 variations in weight, and some unimportant shades of colour. 



"To those, therefore, who admit the radiant form of matter, no dif- 

 ficulty exists in the simplicity of the properties it possesses, but rather 

 an argument in their favour. These persons show you a gradual 

 resignation of properties in the matter we can appreciate as the matter 

 ascends in the scale of forms, and they would be surprised if that effect 

 were to cease at the gaseous state. They point out the greater exer- 

 tions which Nature makes at each step of the change, and think that, 

 consistently, it ought to be greatest in the passage from the gaseous to 

 the radiant form." Life and Letters of Faraday, vol. i., p. 308. 



