[ *33 ] 
The Proof of the firft Part of this Proportion will 
confift, chiefly, in (hewing what Effedl of the Sun it is 
which deprives the Skin of its white Colour, rather than 
what it is which caufes it be black ; for, to prove 
the Caufe of Blacknefs, is the fame as to prove a 
Negative j Blacknefs being a Negative with regard to 
Colour. The Skin then is deprived of its white 
Colour, by the Force and Influence of the Sun, 
thefe four Ways : 
1. By being rendered opaque, from aDiftipation of 
its more aqueous and pellucid Juices. The known 
Effedt of the Sun’s Heat, and which will render all 
Bodies opaque: Nam corpora ea, qua funt maxime 
pellucida-,poterunt , occultorum fuorum meatuam eva - 
cuatione , fat is opaca evade re (a). 
2 . By a Concretion of its Veflels and Glandules, 
from this Diffipation of their aqueous Contents, which 
renders the Skin both thicker and denfer, or more cal- 
lous or rigid. For the Skin being defigned as a Defence 
to the other fubcutaneous Parts, as the Epidermis is 
to the Cutis , they both wonderfully accommodate 
themfelves to the Nature and Force of external Inju- 
ries, fo as to become capable to defend the Body 
from them? as we fee in Smiths, &c. conftantly ufed 
to handle hot and hard Things, who have the Skin 
of their Hands become fo thick and hard or cartila- 
ginous by it, as to be able in time to handle even 
hot Irons : And thus it is, in a great meafure, with 
the Skins of Negroes, Indians , &c. conftantly expofed, 
and generally naked, to the fcorching Heat of the Sun 
in a perpetual Summer. 
3- By 
(a) Newton Opt. 1 6. Prop. III. 
