shct. ml] dissertation second. 71 



nearer perfection in I he first case, or verging towards ex- 

 tinction in the second. 



Suppose the thing inquired into were the cause of white- 

 ness in bodies ; an instantia migrans is found in glass, 

 which, when entire, is without colour, but, when pulveriz- 

 ed, becomes white. The same is the case with water 

 unbroken, and water dashed into foam. In both cases, 

 the separation into particles produces whiteness. So also 

 the communication of fluidity to metals by the application 

 of heat; and (he destruction of that fluidity by the ab- 

 straction of heat, are examples of both kinds of the instan- 

 tia migrans. Instances of this kind are very powerful 

 for reducing the cause inquired after into a narrow space, 

 and for removing all the accidental circumstances. It is 

 necessary, however, as Bacon 1 very justly remarks, that 

 we should consider not merely the case when a certain 

 quality is lost, and another produced, but the gradual 

 changes made in those qualities during their migration, viz* 

 the increase of the one, and the corresponding diminution 

 of the other. The quantity which changes proportionally 

 to another, is connected with it either as cause and effect, 

 or as a collateral effect of the same cause. When, again, 

 we find two qualities which do not increase proportionally, 

 they afford a negative instance, and assure us that the two 

 are not connected simply as cause and effect. 



The mineral kingdom is the great theatre of the inslan- 

 tiae migrantes, where the same nature is seen in all gra- 

 dations, from the most perfect state, lill it become entire- 

 ly evanescent. Such are the shells which we see so perfect 

 in figure and structure in limestone, and gradually losing 

 themselves in the finer marbles, till they can no longer be 

 distinguished. The use, also, of one such fact to explain 



1 Nov. Org. Lib. ii. Aph. 23. 



