rocT. ii.] DISSERTATION SECOND. 79 



crease of volume, except in a very few cases, and in cir- 

 cumstances very particular. 



The most perfect instant ia comitatus known, as being 

 without any negative, is that of body and weight. What- 

 ever is impenetrable and inert, is also heavy in a degree 

 proportional to its inertia. To this there is no exception, 

 though we do not perceive the connexion as necessary. 



Hostile instances, or those of perpetual separation, are 

 the reverse of the former. 



Examples of this are found in air, and the other elastick 

 fluids, which cannot have a solid form induced on them by 

 any known means, when not combined with other substan- 

 ces. So also in solids, transparency and malleability are 

 never joined, and appear to be incompatible, though it is 

 not obvious for what reason. 



IX. Passing over several classes which seem of infe- 

 riour importance, we come to the instantia cruris, the 

 division of this experimental logick which is most frequentr 

 ly resorted to in the practice of inductive investigation. 

 When, in such an investigation, the understanding is plac- 

 ed in equilibrio, as it were, between two or more causes, 

 each of which accounts equally well for the appearances, 

 as far as they are known, nothing remains to be done, but 

 to look out for a fact which can be explained by the one 

 of these causes, and not by the other ; if such a one can be 

 found, the uncertainty is removed, and the true cause is 

 determined. Such facts perform the office of a cross, 

 erected at the separation of two roads, to direct the tra- 

 veller which he is to take, and, on this account, Bacon 

 gave them the name of instantiae cruris. 



Suppose that the subject inquired into were the motion 

 of the planets, and that the phenomena which first present 

 themselves, or the motion of these bodies in longitude, 

 could be explained equally on the Ptolemaick and the Co» 



