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real divisions of nature, and measures of things, and the " how 

 far " nature effects or allows of anything, and her passage 

 thence to something else. Such are gold in weight, iron in hard- 

 ness, the whale in the size of animals, the dog in smell, the flame 

 of gunpowder in rapid expansion, and others of a like nature. 

 Nor are we to pass over the extremes in defect, as well as in 

 abundance, as spirits of wine in weight, the touchstone in soft- 

 ness, the worms upon the skin in the size of animals, and the like. 



35. In the thirteenth rank of prerogative instances, we will 

 place those of alliance or union. They are such as mingle and 

 unite natures held to be heterogeneous, and observed and 

 marked as such in received classifications. 



These instances show that the operation and effect, which is 

 considered peculiar to some one of such heterogeneous natures, 

 may also be attributed to another nature styled heterogeneous, 

 so as to prove that the difference of the natures is not real nor 

 essential, but a mere modification of a common nature. They 

 are very serviceable, therefore, in elevating and carrying on the 

 mind, from differences to genera, and in removing those phan- 

 toms and images of things, which meet it in disguise in concrete 

 substances. 



For example: let the required nature be heat. The classi- 

 fication of heat into three kinds, that of the celestial bodies, that 

 of animals, and that of fire, appears to be settled and admitted ; 

 and these kinds of heat, especially one of them compared with 

 the other two, are supposed to be different, and clearly hetero- 

 geneous in their essence and species, or specific nature, since the 

 heat of the heavenly bodies and of animals generates and cher- 

 ishes, whilst that of fire corrupts and destroys. We have an 

 instance of alliance, then, in a very common experiment, that of 

 a vine branch admitted into a building where there is a constant 

 fire, by which the grapes ripen a whole month sooner than in 

 the air; so that fruit upon the tree can be ripened by fire, al- 

 though this appear the peculiar effect of the sun. From this 

 beginning, therefore, the understanding rejects all essential dif- 

 ference, and easily ascends to the investigation of the real dif- 

 ferences between the heat of the sun and that of fire, by which 

 their operation is rendered dissimilar, although they partake of 

 a common nature. 



These differences will be found to be four in number, i. The 

 heat of the sun is much milder and gentler in degree than that of 

 fire. 2. It is much more moist in quality, especially as it is 

 transmitted to us through the air. 3. Which is the chief point, 

 it is very unequal, advancing and increased at one time, retiring 

 and diminished at another, which mainly contributes to the 

 generation of bodies. For Aristotle rightly asserted that the 

 principal cause of generation and corruption on the surface of 



