30 FIRST PRINCIPLES. 



nosis is due to the poor protection against over-stimulation by 

 sunlight, afforded by the white coat to the cells which form 

 pigment. 



Dr. Pembrey suggests to me, that the role which pigment 

 plays in protecting the body from the direct rays of the 

 sun, consists in preventing their deeper penetration. It would 

 then act in a manner somewhat similar, though of course far 

 less effectively, to that in which a blanket wrapt round a block 

 of ice checks the tendency of the ice to melt during warm 

 weather (p. 31). It would also shelter the sweat nerves from 

 the injurious action which very bright light might have on 

 them. On page 21 I have alluded to the powerful influence 

 which intense light exerts on the nerves. The treatment of 

 lupus by means of the luminous rays of the sun concentrated 

 by a lens, will be suggestive to medical readers. Dr. Pembrey 

 also points out that sun-burn is a protection against the further 

 action of the sun's rays, and that farmers in the South of 

 England generally prefer black pigs to white pigs ; because 

 they find that the former are less affected by a hot sun, than 

 the latter. When going by sea from England to tropical 

 countries, I have often remarked that exposed parts such as 

 the face, neck and arms of fair-skinned sailors or passengers 

 were much more liable to become blistered by the sun, than 

 those of their dark-skinned ship-mates. Although we have 

 undoubtedly strong grounds for believing that pigment pro- 

 tects the body against heat, this supposition has not up to 

 the present been proved by experiment. 



CONDUCTION OF HEAT. 



If a piece of charcoal or stick of sulphur is burning only at 

 one end, we shall find that we can painlessly lift it up by the 

 other end with our fingers ; because charcoal and sulphur are 

 bad conductors of heat ; that is to say, their respective^ powers 

 of transmitting heat through their mass are feeble. On the 



