VOL. XLIII.J PHILOSOPHICAL TRANSACTIONS. 5^ 



countries is the principal, if not the only agent, in producing this effect: but as 

 the authors of this opinion seem not to have understood what effect or altera- 

 tion is produced in the make of the skin, in order to render it black, so they 

 have not been able to satisfy any one in this point, and far less to vindicate their 

 opinion from any material objections ; for it would be very difficult, if not impos- 

 sible, to show or prove, in what manner the sole effect of blackness is occasioned 

 by the power of the sun, but not so difficult to show how it may make the skin 

 thicker or denser, which we shall endeavour in the next place to do, by showing 

 in what manner the power of the sun is able to cause that thickness and density 

 of skin, which we have assigned as the immediate cause of its black or tawny 

 colour. But as this subject is much plainer, and more obvious, than the other 

 concerning the make of the skin, or the immediate cause by which its blackness 

 is occasioned, so we need only give the principal heads of these arguments, which 

 serve to prove this proposition: which arguments are of two sorts, viz. philoso- 

 phical and historical. 



The proof of the first part of this proposition will consist chiefly, in showing 

 what effect of the sun it is which deprives the skin of its white colour, rather 

 than what it is which causes it to be black ; for, to prove the cause of blackness, 

 is the same as to prove a negative; blackness being a negative with regard to 

 colour. The skin then is deprived of its white colour, by the force and influence 

 of the sun, these four ways : 1 . By being rendered opaque, from a dissipation of 

 its more aqueous and pellucid juices. The known effect of the sun's heat, and 

 which will render all bodies opaque. 



1. By a concretion of its vessels and glandules, from this dissipation of their 

 aqueous contents, which renders the skin both thicker and denser, or more cal- 

 lous or rigid. For the skin being designed as a defence to the other subcuta- 

 neous parts, as the epidermis is to the cutis, they both wonderfully accommodate 

 themselves to the nature and force of external injuries, so as to become capable 

 to defend the body from them, as we see in smiths, &c. constantly used to handle 

 hot and hard things, who have the skin of their hands become so thick and hard 

 or cartilaginous by it, as to be able in time to handle even hot irons : and thus it 

 is, in a great measure, with the skins of negroes, Indians, &c. constantly ex- 

 posed, and generally naked, to the scorching heat of the sun in a perpetual 

 summer. 



3. By a new accretion of many new membranes, which render it thick and 

 opaque. For the sun-beams act as a vibrating force, or external friction, on the 

 skin, which derives fresh supplies of juices to it; by which new membranes, or 

 lamellae, are formed, in the same manner as the epidermis is renewed when 

 abraded, which is very soon and easily done. Which derivation and concoction 



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