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easily observed by night than by day: for contemplations of 

 this kind may be considered as carried on by night, from the 

 minuteness and perpetual burning of our watch-light. 



The same must be attempted with inanimate objects, which 

 we have ourselves done by inquiring into the opening of liquids 

 by fire. For the mode in which water expands is different from 

 that observed in wine, vinegar, or verjuice, and very different, 

 again, from that observed in milk and oil, and the like ; and this 

 was easily seen by boiling them with slow heat, in a glass ves- 

 sel, through which the whole may be clearly perceived. But we 

 merely mention this, 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. 



42. 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 

 cannot be obtained. This substitution is twofold, either 1 by 

 approximation 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 sus- 

 ceptible of heat, when brought near the fire; yet air becomes 

 warm much sooner than stone. These are examples of sub- 

 stitution 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 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 

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

 stances ; 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 

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