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twofold, either by approximation or by analogy. For in 

 stance, 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 approximation. 



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 impercep 

 tible body, but by the consideration of some similar per 

 ceptible 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 substances; for flame 

 increases over the exhalation of oil, and air over that of 

 water. One must therefore consider the mixture of oil and 

 water, which is manifest to the senses, since that of air and 

 flame in general escapes the senses. But oil and water mix 

 very imperfectly by composition or stirring, while they are 

 exactly and nicely mixed in herbs, blood, and the parts of 

 animals. Something similar, therefore, may take place in 



