Radiation, in the sense used here, is the transmission of rays of heat, or rays of heat and light, by a body without raising the temperature of the surrounding air. All bodies, no matter what their temperature may be, constantly radiate heat; the loss thus entailed being more or less made up by absorption of radiated heat from other bodies, and in some cases by conduction of heat from bodies in contact with them. The rays of the sun are composed chiefly of rays of light, and, to a smaller extent, of rays of heat; the former being capable of becoming converted into the latter and vice versa. If, during summer in a hot country, we ascend a mountain, say to a height of 16,000 ft., we shall find that the rays of the sun are as powerful as in the plains, but that the temperature of the air is below freezing point. We may therefore conclude in such a case, that the heat of the air in the plains is due to contact with the ground, buildings, and other objects which have become heated by the rays of the sun; but not to radiation. Heat received by means of radiation is partly absorbed and partly reflected. Thus, when a horse is placed in a cold stall, his body by conduction (p. 30) raises the temperature of the air which surrounds it, and radiates heat that warms the partitions, walls, etc. upon which it falls. These objects in their turn warm the air with which they are in contact, and radiate heat to other objects, including the horse, until finally the temperature of the atmosphere of the stall becomes more or less raised, and more or less equalised. In the open, almost all the heat radiated by a horse is lost, as far as he is concerned, in his surroundings, and consequently during cold weather the temperature outside is lower than in the stable.

The light rays of the sun (luminous rays) are capable of easily going through ordinary window-glass, which offers a considerable obstacle to the passage of the heat rays (obscure rays). Consequently, when sunshine falls on the panes of a closed stable window, the heat rays are more or less taken up by the glass, and the light rays enter the 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.