VOL. XLIII.] PHILOSOPHICAL TRANSACTIONS. 55 



extract whatever juices they might contain, as we see plainly they do, by the large 

 blisters raised by such applications, from the abraded vessels spewing out their 

 liquid contents; in which blisters there are no tokens of any black humours in 

 negroes more than in whites, as he often found on proper trials; though if there 

 was any such black humour contained in their subcutaneous vessels, there is no 

 doubt but it would appear, in some measure, in the water of their blisters, as well 

 as the yellow bile does, when diffused through the blood, and on the skin. 



Prop. v. — The epidermis, especially its external lamella, is divided into two 

 parts, by its pores and scales, two hurulred times less than the particles of bodies, 

 on rvhich their colours depend. — Sir Isaac Newton informs us, that the particles 

 of bodies, on which their colours depend, are about 600 times less than those 

 which can be discerned with the naked eye; Opt. lib. 1, part 3, prop. 7. But 

 Leuwenhoeck shows, that a portion of the epidermis, no larger than what can 

 be discerned with the naked eye, is divided into 125000 pores; which pores must 

 divide such a portion of the skin as can be discerned with the naked eye, into 

 125000 particles; therefore each of these parts of the skin, between its pores, 

 must be about 200 times less than those particles, on which the colours of bodies 

 depend; for ' ^-i^" = 208^; not to mention that such a portion of the epi- 

 dermis is further divided into 250 scales, which must increase the number of 

 parts into which it is divided. Nor will any one think, that the smallness of 

 these parts and pores exceeds credibility, who considers that they convey the mi- 

 nutest particles of our last digestions; and were they even large enough to convey 

 the particles of many waters, it is very probable, that all our fluids might in 

 time evaporate through them. Nor is it any thing to our present purpose, whe- 

 ther these numbers be mathematically exact or not, all that is endeavoured to 

 be proved, is, that the parts into which the cuticle is divided, are less, in some 

 proportion, than the particles of bodies on which their colours depend. 



Prop. vi. — Problem: To determine and explain the proximate cause of the 

 colour of Negroes, Indians, white people, ^c. from the foregoing propositions. — 

 We have shown above, prop. 1 , that the colour of white people depends on the 

 colour which the epidermis transmits, and not on what it reflects: this pellucidity 

 of bodies proceeds from the number of interstices between the particles which 

 compose them, and the extreme smallness of those particles ; for, in order to 

 render a body of any colour, or fit to reflect the rays of light, its particles, and 

 the intervals between them, ought not to be less than a certain magnitude; else 

 they become incapable of making any reflexions, from their common surfaces, 

 i.e. of appearing coloured: but, by prop. 5, the cuticle is divided into parts, and 

 pores or intervals between these particles, far less than those on which the colours 

 of bodies depend; that is, too small to reflect any rays of light from their common 

 surfiices, or to appear coloured from such reflected rays: but as such porous 



