TEMPERATURE OF SPACE. 261 



distance, I have hitherto adopted the former mode. 

 This probably makes the change of temperature too 

 great, while Dulong's and Petit's formula, adopted by 

 Mr. Hill (" Nature," vol. xx. p. 626), on the other hand, 

 makes it too small. Dulong's and Petit's formula is an 

 empirical one, which has been found to agree pretty 

 closely with observation within ordinary limits, but 

 we have no reason to assume that it will hold equally 

 correct when applied to that of space, any more than 

 we have to infer that it will do so in reference to tem- 

 perature as high as that of the sun. When applied to 

 determine the temperature of the sun from his rate of 

 radiation, it completely breaks down, for it is found to 

 give only a temperature of 2130° F. ("Amer. Jour. 

 Science," July, 1870), or not much above that of an 

 ordinary furnace. 



But besides all this it is doubtful if it will hold true 

 in the case of gases. From the experiments of Prof. 

 Balfour Stewart (" Trans. Edin. Boy. Soc", xxii.) on the 

 radiation of glass plates of various thicknesses, it would 

 seem to follow that the radiation of a material particle 

 is probably proportionate to its absolute temperature, 

 or, in other words, that it obeys Newton's law. Prof. 

 Balfour Stewart found that the radiation of a thick 

 plate of glass increases more rapidly than that of a 

 thin plate as the temperature rises, and that, if we go 

 on continally diminishing the thickness of the plate 

 whose radiation at different temperatures we are ascer- 

 taining, we find that, as it grows thinner and thinner, 

 the rate at which it radiates its heat as its temperature 

 rises becomes less and less. In other words, as the 

 plate grows thinner its rate of radiation becomes more 

 and more proportionate to its absolute temperature. 

 And we can hardly resist the conviction that if it were 

 possible to go on diminishing the thickness of the plate 



