We learn from the study of physics that the radiating power of the body is equal to its heat absorbing power, and that its heat absorbing power is in inverse proportion to its heat reflecting power. Hence, bodies that reflect heat well, like those which are polished, radiate heat feebly. We therefore find that a horse with a gloss on his coat, other things being equal, will bear exposure to cold better than an animal with a dull coat. Also, light-coloured surfaces radiate heat slower than dark ones, and we have every reason to believe that the change from a dark colour to a light one which the coats of many Arctic animals undergo on the approach of winter, acts beneficially in protecting them from cold, as well as in adapting them to their surroundings. The observations I have made in cold countries lead me to think that light-coloured horses usually stand cold better than dark ones, but having no exact data to offer, I give the opinion merely for what it is worth.

I have frequently noticed in hot climates, that white horses which have little or no pigment in their skin (equine albinos' as we might term them) bear exposure to hot sunshine far worse than other kinds of horses. I may explain to readers who have not made a special study of horses, that the great majority of white and grey horses, and all coloured horses, have black skins. I have also observed, though not to such a convincing degree, that the darker a horse's coat is, the better, as a rule, will he ward off the bad effects of the sun. General Daumas in his book, Les Chevaux du Sahara, was, I believe, the first to record this supposed fact, which he states is accepted by the horsemen of the Algerian desert. It appears that the presence of pigment in the skin of negroes and other inhabitants of tropical climates helps to protect the body, when it is exposed to severe solar heat. We learn from experience that in such a case, the skin of a black man, under similar conditions of health, will feel colder to the touch than that of a European or an American. We know that sunlight plays an essential part in the formation of pigment, especially in plants; and it is significant that the colouring of tropical animals as a rule is darker and more brilliant than that of Arctic animals. In horses, melanotic tumours, which are characterised by the formation of a very large amount of pigment, are with few exceptions confined to grey horses, and in such cases they almost always appear after the animal is at least nine years old, at which age the coat begins to turn white. Also, although I have no statistics to bear out the assertion, I feel convinced, from personal observation, that melanosis is a much more common disease among grey horses in hot climates, than in cold or temperate ones. We may reasonably hazard the conjecture that susceptibility to melanosis 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 protects the body against heat, this supposition has not up to the present been proved by experiment.