Prof. J. C. Maxwell on Molecules. 465 



This of course is Dot the only method of studying human 

 nature. We may observe the conduct of individual men and 

 compare it with that conduct which their previous character and 

 their present circumstances, accordiug to the best existing 

 theory, would lead us to expect. Those who practise this 

 method endeavour to improve their knowledge of the elements 

 of human nature in much the sauie way as an astronomer cor- 

 rects the elements of a planet by comparing its actual position 

 with that deduced from the received elements. The study of 

 human nature by parents and schoolmasters, by historians and 

 statesmen is therefore to be distinguished from that carried on 

 by registrars and tabulators, and by those statesmen who put 

 their faith in figures. The one may be called the historical, and 

 the other the statistical method. 



The equations of dynamics completely express the laws of the 

 historical method as applied to matter ; but the application of 

 these equations implies a perfect knowledge, of all the data. 

 But the smallest portion of matter which we can subject to ex- 

 periment consists of millions of molecules, not one of which ever 

 becomes individually sensible to us. We cannot, therefore, ascer- 

 tain the actual motion of any one of these molecules ; so that we 

 are obliged to abandon the strict historical method and to 

 adopt the statistical method of dealing with large groups of 

 molecules. 



The data of the statistical method as applied to molecular 

 science are the sums of large numbers of molecular quantities. 

 In studying the relations between quantities of this kind we 

 meet with a new kind of regularity, the regularity of averages, 

 which we can depend upon quite sufficiently for all practical 

 purposes, but which can make no claim to that character of ab- 

 solute precision which belongs to the laws of abstract dynamics. 



Thus molecular science teaches us that our experiments can 

 never give us any thing more than statistical information, and 

 that no law deduced from them can pretend to absolute preci- 

 sion. But when we pass from the contemplation of our experi- 

 ments to that of the molecules themselves, we leave the world 

 of chance and change, and enter a region where every thing is 

 certain and immutable. 



The molecules are conformed to a constant type with a preci- 

 siou which is not to be found in the sensible properties of the 

 bodies which they constitute. In the first place, the mass of 

 each individual molecule and all its other properties are abso- 

 lutely unalterable. In the second place, the properties of all 

 molecules of the same kind are absolutely identical. 



Let us consider the properties of two kinds of molecules, 

 those of oxygen and those of hydrogen. 



Phil. Mag. S. 4. Vol. 46. No. 308. Dec. 1873. 2 I 



