222 Newton's Letters to Oldenburg and Boyle. 



are reduced back into water, unless the heat, which keeps 

 them, in agitation, be so great as to dissipate them as fast as 

 they come together; but the grosser particles of exhalations 

 raised by fermentation keep their aerial form more obstinately, 

 because the aether within is rarer. 



Nor does the size only, but the density of the particles also, 

 conduce to the permanency of aerial substances ; for the ex- 

 cess of density of the aether without such particles above that 

 of the aether within them is still greater ; which has made me 

 sometimes think that the true permanent air may be of a me- 

 tallic original ; the particles of no substances being more 

 dense than those of metals. This, I think, is also favoured 

 by experience, for I remember I once read in the Philoso- 

 phical Transactions, how M. Huygens at Paris, found that 

 the air made by dissolving salt of tartar would in two or three 

 days' time condense and fall down again, but the air made by 

 dissolving a metal continued without condensing or relenting 

 in the least. If you consider then, how by the continual fer- 

 mentations made in the bowels of the earth there are aerial 

 substances raised out of all kinds of bodies, all which together 

 make the atmosphere, and that of all these the metallic are 

 the most permanent, you will not perhaps think it absurd, 

 that the most permanent part of the atmosphere, which is the 

 true air, should be constituted of these, especially since they 

 are the heaviest of all others, and so must subside to the lower 

 parts of the atmosphere and float upon the surface of the 

 earth, and buoy up the lighter exhalations and vapours to 

 float in greatest plenty above them. Thus, I say, it ought to 

 be with the metallic exhalations raised in the bowels of the 

 earth by the action of acid menstruums, and thus it is with 

 the true permanent air ; for this, as in reason it ought to be 

 esteemed the most ponderous part of the atmosphere, because 

 the lowest, so it betrays its ponderosity by making vapours 

 ascend readily in it, by sustaining mists and clouds of snow, 

 and by buoying up gross and ponderous smoke. The air also 

 is the most gross unactive part of the atmosphere, affording 

 living things no nourishment, if deprived of the more tender 

 exhalations and spirits that float in it; and what more unac- 

 tive and remote from nourishment than metallic bodies? 



I shall set down one conjecture more, which came into my 

 mind now as I was writing this letter ; it is about the cause 

 of gravity. For this end I will* suppose aether to consist of 

 parts differing from one another in subtilty by indefinite de- 

 grees; that in the pores of bodies there is less of the grosser 

 aether, in proportion to the finer, than in open spaces ; and 

 consequently, that in the great body of the earth there is much 



