CONTRIBUTIONS TO SCIENCE. 571 



Where s = distance of centres at striking. 

 1ST Number in cubic centimetre. 

 V Mean velocity. 



Now ^-s 2 N - 17,700 for air, and V = 48,500. 



But this is in electrostatic measure. In electromagnetic 

 measure the resistance is 



TT 48500000 



so that r 2.10 13 per cubic centimetre, or about 10 10 greater than 

 that of copper; but this is far smaller than that of gutta- 

 percha. Hence the insulating power of air is not consistent 

 with its molecules being conducting spheres. 



But why should the molecules be conductors ? Yours very 

 truly, J. CLERK MAXWELL. 



Maxwell's investigations in the Kinetic Theory of Gases 

 led him to a conclusion which is of great value in the 

 theory of energy. The principle of the dissipation of 

 energy, sometimes called the second law of Thermodynamics, 

 states that it is impossible by means of inanimate material 

 agency to obtain work at the expense of heat by cooling a 

 body below the temperature of the coldest body in the 

 neighbourhood. This principle was first distinctly given by 

 Sir William Thomson. Maxwell showed that it obtains 

 only in consequence of the coarseness of our faculties not 

 allowing us to grapple with individual molecules. If we 

 could seize upon individual molecules, and bring them to 

 rest in the same way as we can lay hold of a fly-wheel, and 

 compel it to do useful work until it has been deprived of 

 all its motion of rotation, we could convert the whole of the 

 heat of a body into work, and bring it to the absolute zero 

 of temperature. As it is, we are at the mercy of the mole- 

 cules, and capable of obtaining from them only so much 

 work as they are willing to give in the most favourable 

 circumstances in which we are able to place them. Maxwell 

 imagined a quantity of gas, all initially at the same pressure 

 and temperature, to be divided into two portions, A and B, 

 by a partition full of little trap doors which might be 



