28 FIRST PRINCIPLES. 



building. These light rays on coming in contact with the 

 surfaces inside the stable, become converted for the most part 

 into heat rays, which are radiated throughout the building, but 

 are unable to escape outwards, except through an open door 

 or open window, and to a slight extent through the glass, 

 which, on becoming heated by the direct rays of the sun, 

 radiates heat inwards as well as outwards. Hence, the 

 temperature of the air in the stable becomes more and more 

 raised according as the rays of light continue to enter. This 

 heating effect caused by the feeble power which heat rays 

 possess of passing through glass, is well shown in the case 

 of a green-house, the temperature of the atmosphere of 

 which on a sunshiny day will be much higher than that of 

 the air outside. If, therefore, we wish to keep our 

 stables cool, we should avoid the employment of much 

 glass, and should keep the windows open when the sun is 

 shining in their direction. 



PROTECTION AGAINST HEAT BY COLOUR. 



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 there- 

 fore 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 sur- 

 roundings. The observations I have made in cold countries 

 lead me to think that light-coloured horses usually stand cold 



