72 DISSERTATION SECOND. [part ,, 



or interpret another, is nowhere so well seen as in the his- 

 tory of the mineral kingdom. 



III. In the third place are the instantiae ostensivae, 

 which Bacon also calls elucescentiae and predominantes. 

 They are the facts which show some particular nature in 

 its highest state of power and energy, when it is either 

 freed from the impediments which usually counteract it, 

 or is itself of such force as entirely to repress those im- 

 pediments. For as every body is susceptible of many 

 different conditions, and has many different forms combin- 

 ed in it, one of them often confines, depresses, and hides 

 another entirely, so that it is not easily distinguished. 

 There are found, however, some subjects in which the 

 nature inquired into is completely displayed, either by 

 the absence of impediments, or by the predominance of its 

 own power. 



Bacon instances the thermometer, or vitrum calendare, 

 as exhibiting the expansive power of heat, in a manner 

 more distinct and measurable than in common cases. To 

 this example, which is well chosen, the present state of 

 science enables us to add many others. 



If the weight of the air were inquired into, the Torri- 

 cellian experiment or the barometer affords an ostensive 

 instance, where the circumstance which conceals the 

 weight of the atmosphere in common cases, namely, the 

 pressure of it in all directions, being entirely removed, 

 that weight produces its full effect, and sustains the whole 

 column of mercury in the tube. The barometer affords 

 also an example of the instantia migrans, when the change 

 is not total, but only partial, or progressive. If it be the 

 weight of the air which supports the mercury in the tube 

 of the barometer, when that weight is diminished, the 

 mercury ought to stand lower. On going to the top of 

 a mountain, the weight of the incumbent air is diminish- 



