330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



in the fresh sections of skin (cut frozen) when radiated and wet 

 with a sohition of dioxyphenyhihmin ; this substance is said to be 

 the specific precursor (Bloch). The closely allied compound, ty- 

 rosin, is said to diminish in amount in the blood at the time when 

 melanin is being formed in the skin after a light bath. 



Ultra-violet rays act more quickly on warm than on cold skin. 

 Tested on infusoria the coefficient for temperature (for a rise from 

 10° C. to 20° C.) is about 2-3 (A. Eidinow and L. Hill) ; for the 

 frog's mesentery it is less, about 1-2 (Argyll Campbell and L. 

 Hill). While heat of the sun may aggravate a sunburn, it is not 

 a necessary adjuvant. Ultra-violet radiation can intensely burn the 

 cooled skin; it is w^ell known that glacier sunburns may be very 

 severe. Dewar killed microbes with ultra-violet rays at the tempera- 

 ture of liquid air. 



The power behind the sun w^as worshipped by the heretic pharaoh, 

 Akhnaton, and modern science leads us back to veneration of this 

 power. The imagination tries to think of the infinitely intricate 

 energy complex which goes to form a living cell, of electrons being 

 displaced in atoms by iiltro-violet rays, of molecular movement en- 

 hanced by heat rays, of radiation provoking reactions which mani- 

 fest themselves as life, of the spirit of man ensuing in the evolution 

 of energy transformations. 



The law holds that absorption of rays precedes action. Rays 

 which pass through a cell have no action upon it at all. The skin 

 screens itself from excessive light by its horny layer and by pig- 

 ment. Pigment by absorbing visible and ultra-violet rays screens 

 the deeper cells and blood. It absorbs dark heat rays and con- 

 verts visible rays into heat, and this heat, stimulating the nerve 

 endings in the skin, may reflexly lessen body heat production while 

 provoking sweating and dilatation of cutaneous blood vessels. 



Melanin is a screen, not a sensitizer, transforming light into heat. 

 It is present in a fine particulate form and scatters and diffusely 

 reflects rays. The spectrograph shows that melanin in fine sus- 

 pension and thin layers screens off and greatly weakens but does 

 not wholly absorb the ultra-violet spectrum. This is in sharp con- 

 trast to a 3 per cent solution of quinine which in an equally thin 

 layer wholly absorbs rays shorter than 330 fxfi. A layer of 

 sweat wetting the skin helps to reflect light, wliile a laj^er of evapo- 

 rating moisture surrounding the skin helps to absorb heat rays. The 

 pigmented naked body with sweating skin is thus favorable to 

 the cooling of the native in the tropics, while clothing retards heat 

 loss of the white man. 



By local concentration of an arc light on a rabbit's head, it is 

 easy to heat its fur up to 150° F. and its brain even to 107° F., 

 while with the body in the shade the rectum is only 101° F. (Argyll 



