526 ITOVUM ORGANUM. [BOOK n. 



merely mention tins, intending to treat of it more at large and 

 more closely when we come to the discovery of the latent- 

 process ; for it should always be remembered that we do not 

 here treat of things themselves, but merely propose examples. 



XLII. In the nineteenth rank of prerogative instances we 

 will class supplementary or substitutive instances, which we are 

 also wont to call instances of refuge. They are such as supply 

 information, where the senses are entirely deficient, and we 

 therefore have recourse to them when appropriate instances can 

 not be obtained. This substitution is twofola, either by approxi 

 mation or by analogy. For instance, there is no known medium 

 which entirely prevents the effect of the magnet in attracting 

 iron, neither gold, nor silver, nor stone, nor glass, wood, water, 

 oil, cloth, or fibrous bodies, air, flame, or the like. Yet by 

 accurate experiment, a medium may perhaps be found which 

 would deaden its effect, more than another comparatively and in 

 degree ; as, for instance, the magnet would not perhaps attract 

 iron through the same thickness of gold as of air, or the same 

 quantity of ignited as of cold silver, and so on ; for we have not 

 ourselves made the experiment, but it will suffice as an example. 

 Again, there is no known body which is not susceptible of heat, 

 when brought, near the fire ; yet air becomes warm much sooner 

 than stone. These are examples of substitution by approxima 

 tion. 



Substitution by analogy is useful, but less sure, and therefore 

 to be adopted with some judgment. It serves to reduce that 

 which is not the object of the senses to their sphere, not by the 

 perceptible operations of the imperceptible body, but by the 

 consideration of some similar perceptible body. For instance, 

 let the subject for inquiry be the mixture of spirits, which are 

 invisible bodies. There appears to be some relation between 

 bodies and their sources or support. Now, the source of flame 

 seems to be oil and fat ; that of air, water, and watery sub- 



c The itinerant instances, as well as frontier instances, are cases in 

 which we are enabled to trace the general law of continuity which 

 seems to pervade all nature, and which has been aptly embodied in the 

 sentence, &quot;natura non agit per saltum.&quot; The pursuit of this law into 

 phenomena where its application is not at first sight obvious, has 

 opened a mine of physical discovery, and led us to perceive an inti 

 mate connection between facts which at first seemed hostile to each 

 other. For example, the transparency of gold-leaf, which permits a 

 bluish-green light to pass through it, is a frontier instance between 

 transparent and opaque bodies, by exhibiting a body of the glass gene 

 rally regarded the most opaque in nature, as still possessed ot some 

 slight degree of transparency. It thus proves that the quality of 

 opacity is not a contrary or antagonistic quality to that of transparency, 

 but only it* extreme lowest degree. 



