The experiments in the first section have been carried out mainly in two ways. In the first, the rate of cooling of the full radiator has been determined, and from the rate of cooling at any temperature the rate at which heat was lost by radiation was immediately calculated. Newton was the first to investigate in this way by observing the rate at which a thermometer bulb cooled down when it was surrounded by an enclosure which was kept at a uniform temperature. He found that the rate of cooling, and therefore the rate at which heat was lost by the thermometer, was proportional to the difference of temperature between the thermometer and its surroundings. This rule is known as Newton's Law of Cooling, and is still used when it is desired to correct for the heat lost during an experiment where the temperature differences are small. It is only true, however, for very small differences of temperature between the thermometer and its surroundings, and as early as 1740 Martine had found that it was only true for a very limited range of temperature. +Prévost's Theory of Exchanges.+--In 1792, Prévost of Geneva, when endeavouring to explain the supposed radiation of cold, introduced the line of thought, that any body is not to be regarded as radiating heat only when its temperature is falling, or absorbing heat only when its temperature is rising, but that both processes are continually and simultaneously going on. The amount of heat radiated will depend on the temperature and character of the body itself, while the amount absorbed will depend upon the condition of the surroundings as well as upon the nature {47} of the body. If the amount of heat radiated is greater than the amount absorbed the body will fall in temperature, and _vice-versa_. This view of Prévost's is called the Theory of Exchanges, and we can see that it is a necessary consequence of our ideas as to the production of heat and light waves by the agitation of electrons in the radiating body. If the rate of cooling of a body at a certain temperature is measured when it is placed in an enclosure at a lower temperature, it must be borne in mind that the rate of loss of heat is equal to the rate at which heat is radiated minus the rate at which it is absorbed from the enclosure. A second way in which the heat lost by a body has been measured at different temperatures is by heating a conductor such as a thin platinum strip by means of an electric current, and measuring the temperature to which the conductor has attained. When its temperature is steady, all the energy given to it by the current must be lost as heat, and therefore the electrical energy, which can very easily be calculated, must be equal to the heat radiated by the body minus the heat received from the enclosure.