22 DISCUSSIONS IN CLIMATOLOGY. 



Law of Dulong and Petit — Prof. Newcomb gives 

 his readers to understand that I assume Newton's laws 

 of cooling to be correct ; and that I apparently nowhere 

 adduce the more correct law of Dulong and Petit — viz., 

 that if we take a series of temperatures in arithmetical 

 progression, the corresponding rates of radiation of 

 heat will not be in arithmetical progression, but in a 

 series of which the differences continually increase. If 

 he will refer to the "Header," Dec. 9, 1865, "Phil. Mag.," 

 Feb. 1870, "Nature," April 1, 1880, and 'Climate and 

 Time' (the book he reviewed), p. 37, he will there see 

 the question discussed at considerable length. He will 

 also find reference made to a remarkable circumstance 

 connected with radiation which perhaps may be new 

 to him. It is this : the law of Dulong and Petit (that 

 as the temperature of a body rises the radiation of the 

 body increases in a much higher ratio) holds true only 

 of the body considered as a mass. The probability is, 

 as has been shown by Prof. Balfour Stewart, that the 

 individual particles composing the body obey Newton's 

 law in their radiation ; in other words, the radiation of 

 a material particle is directly proportionate to its abso- 

 lute temperature. 



Further, in estimating the extent to which tempera- 

 ture is affected by a change in the sun's distance, 

 Newton's law makes the extent too great ; while the 

 formula of Dulong and Petit, which is an empirical 

 one, makes it, on the other hand, too small. This 

 formula has been found to agree pretty closely with 

 observation within ordinary limits, but it completely 

 breaks down when applied to determine high tempera- 

 tures. For example, it is found to give a temperature 

 for the sun of only 2130° F., or not much above that of 

 an ordinary furnace. It is probable also that it will 

 equally break down when applied to very low tem- 

 peratures, such as that of space. 



