THE THEORY OF MOLECULES. 287 



The number of individuals is far too great to allow of their tracing the 

 history of each separately, so that, in order to reduce their labor within 

 human limits, they concentrate their attention on a small number of 

 artificial groups. The varying number of individuals in each group, 

 and not the varying state of each individual, is the primary datum 

 from which they work. 



This, of course, is not 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 circum- 

 stances, according to the best existing theory, would lead us to expect. 

 Those who practise this method endeavor to improve their knowledge 

 of the elements of human nature in much the same way as an astrono- 

 mer corrects 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 school-masters, 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 his- 

 torical method as applied to matter, but the application of these equa- 

 tions implies a perfect knowledge of all the data. But the smallest 

 portion of matter which we can subject to experiment consists of mill- 

 ions of molecules, not one of which ever becomes individually sensible 

 to us. We cannot, therefore, ascertain the actual motion of any one 

 of these molecules, so that we are obliged to abandon the strict his- 

 torical 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 absolute 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 precision. But, when we 

 pass from the contemplation of our experiments to that of the mole- 

 cules 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 precision 

 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 mole- 

 cule, and all its other properties, are absolutely unalterable. In the 

 second place, the properties of all molecules of the same kind are ab- 

 solutely identical. 



