60 PHILOSOPHICAL TRANSACTIONS. [aNNO 1744. 



of the humours on the surface of the body must occasion a thickness of their 

 skins, as well as of their lips, and other muscles, especially of their face. 



4. By increasing those parts or principles, in the composition of the epidermis, 

 which have the greatest refractive powers. As the terrestrial, and fixed saline; 

 but, especially, the tenacious sulphureous, which refract and absorb light more 

 strongly than any other substances ; while the more transparent and pellucid prin- 

 ciples, as the aqueous, spirituous, and volatile saline, are evaporated by the heat, 

 which causes the other more fixed principles to be accumulated in greater quan- 

 tities, and combined in larger collections ; and these particles, being likewise more 

 comminuted by the sun, will on that account be black, as happens to oil when 

 well boiled. 



From what we have said above about the immediate causes of the colours of 

 the skin, it will appear, that these several effects of the sun's heat contribute to 

 make it of a darker colour; and no one will doubt, but that all of them, con- 

 spiring together, may make it quite black. 



To these, perhaps, might be added another effect of the sun's power, a pecu- 

 liar necrosis of the epidermis, occasioned by the forcible vibrations, contractions, 

 and exsiccations of its fibres by the sun-beams, which cause it to turn black, as 

 these, or the other parts, do by the heat of an inflammation or a fever, in gan- 

 grenes, black tongues, &c. Whence only the nervous parts of the skin come 

 to be black, and more hard and callous, and less pellucid, than the rest; and 

 the skins of negroes, besides their callosity, become more insensible than those 

 of whites. 



But as there are many degrees of whiteness and blackness in the colours of the 

 people in the world, depending on the different densities and thicknesses of their 

 respective cuticles, as we have above showed, it may not be improper, in the 

 next place, to inquire into the more peculiar causes of this diversity, which will 

 be found to be such as increase or diminish the power of the sun's heat, or its 

 influence on the body; by which the only material objection that has been brought 

 against this proposition may be answered, viz. that the sun's heat is not the cause 

 of negroes, because several nations of people, in the same latitude with the 

 negroes in Africa, are not made black by it. 



The causes of this diversity may be referred to two heads, viz. 1. The nature 

 and temper of the country. 2. The ways of living in it. Under the first may 

 be included the following particulars : 



1. The nature of the soil, and situation of the country, with regard to moun- 

 tains, waters, &c. which very much alter the power of the sun's heat; for the 

 differing degrees of heat and cold, in different places, depend in a great measure 

 on the accidents of the neighbourhood of high mountains, whose height exceed- 



