NOTES. 



NOTE A. ART. 1. 



WHEN, as in the present subject, and in the cognate theory of 

 Elastic Solids, we study the changes of shape which a mass of matter 

 undergoes, we begin by considering the whole mass as made up of a 

 large number of very small parts, or elements, and endeavouring to 

 take account of the motion, or the displacement, of each of these. 



If we inquire however to what extent this ideal subdivision is to be 

 carried, we are at once brought face to face with questions as to the 

 ultimate structure and properties of matter. We have, in the text, 

 adopted the hypothesis of a continuous structure, in which case the 

 subdivision is without limit. 



The truth of this hypothesis is, however, not probable. It is now 

 generally held that all substances are ultimately of a heterogeneous or 

 coarse-grained* structure; that they are in fact built up of discrete 

 bodies or molecules, separated, it may be, by more or less wide 

 intervals. These bodies are far too minute to admit of direct obser 

 vation, so that we are almost wholly ignorant of their nature and of 

 the manner in which they act 011 one another. It would therefore be 

 futile to attempt to form the equations of motion of individual molecules, 

 and even if the equations could be formed and integrated the results 

 would not be directly comparable with observation, nor would they even 

 be of interest from the point of view of our present subject. For our 

 object is not to follow the careers of individual molecules, which cannot 

 be traced or identified, but to study the motions of portions of matter 

 which, though very small, are still large enough to be observed, and 



* Thomson and Tail, Natural Philosophy, Art. 675. 



