216 Newton's Letters, Hypothesis and Experiments 



stands sometimes up to the top of a glass pipe, though much 

 higher than 30 inches ; and one of the main causes why the 

 parts of all bodies cohere; also the cause of filtration, and of 

 the rising of water in small glass pipes above the surface of the 

 stagnating water they are dipped into; for I suspect the aether 

 may stand rarer, not only in the insensible pores of bodies, 

 but even in the very sensible cavities of the pipes ; and the 

 same principles may cause menstruums to pervade with vio- 

 lence the pores of the bodies they dissolve, the surrounding 

 aether, as well as the atmosphere, pressing them together. 



3. I suppose the rarer aether within bodies, and the denser 

 without them, not to be terminated in a mathematical super- 

 ficies, but to grow gradually into one another; the external 

 aether beginning to grow rarer, and the internal to grow 

 denser, at some little distance from the superficies of the 

 body, and running through all intermediate degrees of den- 

 sity in the intermediate spaces ; and this may be the cause why 

 light, in Grimaldo's experiment, passing by the edge of a 

 knife, or other opake body, is turned aside, and as it were 

 refracted, and by that refraction makes several colours. Let 

 ABCD be a dense body whe- jpjg. i. 



ther opake or transparent, 

 EFGH the outside of the S< ^x 



uniform aether, which is within n^^ 



it, IKLM the inside of the ,*— ^^ 



uniform aether, which is with- 

 out it; and conceive the aether, 

 which is between EFGH and 

 IKLM, to run through all 

 intermediate degrees of den- 

 sity between that of the two 

 uniform aethers on either side. 

 This being supposed, the rays / 



of the sun SB, SK, which pass by the edge of this body be- 

 tween B and K, ought in their passage through the unequally 

 dense aether there, to receive a ply from the denser aether, 

 which is on that side towards K, and that the more by how 

 much they pass nearer to the body, and thereby to be scat- 

 tered through the space PQRST, as by experience they are 

 found to be. Now the space between the limits EFGH and 

 IKLM, I shall call the space of the aether's graduated rarity. 



4>. When two bodies moving towards one another come 

 near together, I suppose the aether between them to grow 

 rarer than before, and the spaces of its graduated rarity to 

 extend further from the superficies of the bodies towards one 

 another ; and this, by reason that the aether cannot move and 



