206 DISCOURSE ON THE STUDY 



dually, as the force by which they are pressed 

 together increases, till they actually cohere, and 

 form one. The law of continuity, then, of which 

 we have before spoken ($ 199.), forbids us to sup- 

 pose that the intimate nature of the process of 

 communication is changed in this transition from 

 light to violent contact, and from that to actual 

 union. If so, we might ask, at what point does the 

 change happen? Especially since it is also de- 

 monstrable, that the particles of the most solid 

 body are not, really, in contact. Therefore, the 

 laws of conduction and radiation have a mutual 

 dependence, and the former are only extreme cases 

 of the latter. If, then, we would rightly understand 

 what passes, or what is the process of nature in 

 the slow communication of heat through the sub- 

 stance of a solid, we must ground our enquiries 

 upon what takes place at a distance, and then urgfe 

 the laws to which we have arrived, up to their 

 extreme case. 



(218.) When two theories run parallel to each 

 other, and each explains a great many facts in com- 

 mon with the other, any experiment which affords a 

 crucial instance to decide between them, OF by 

 which one or other must fall, is of great importance. 

 In thus verifying theories, since they are grounded 

 on general laws, we may appeal, not merely to par- 

 ticular cases, but to whole classes of facts ; and we 

 therefore have a great range among the individuals 

 of these for the selection of some particular effect 

 which ought to take place oppositely in the event 

 of one of the two suppositions at issue being right 

 and the other wrong. A curious example is given 



