Professor Newcomb's "Rejoinder " 277 



posing the body obey Newton's law in their radiation; in 

 other words, the radiation of a material particle is directly 

 proportionate to its absolute temperature. 



Further, in estimating the extent to which temperature 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 com- 

 pletely 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 temperatures, such as that of space. 



I am very much pleased to find that Prof. Newcomb draws 

 a conclusion from Dulong and Petit's law favourable to my 

 theory of the cause of the Glacial Epoch, which certainly did 

 escape my notice. And it is a curious circumstance that Mr. 

 Hill* has likewise deduced a conclusion even more favourable 

 to glaciation than that of Prof. Newcomb. 



Prof. Newcomb says: — " Mr. Croll suggests that I may have 

 forgotten the researches of Pouillet and Herschel into the 

 temperature of space. I reply that I regard the conclusion 

 that the temperature of space is —239° as having no sound 

 basis." This may be perfectly true ; but it is hardly a war- 

 rant for affirming that practically there is but one source (the 

 sun) from which the surface of the earth receives heat, with- 

 out even referring to the researches of these eminent physicists, 

 who have arrived at a totally different conclusion. Any one 

 who has read ' Climate and Time ' will know that I adopted 

 — 239° as the temperature of space, not because I believed 

 that estimate to be correct, but because at the time I wrote 

 there was no other to adopt. In fact in adopting so high a 

 temperature for space I was doing my theory a positive injury. 

 This is obvious; for the lower the temperature of space the 

 greater must be the decrease of temperature resulting from an 

 increase in the sun's distance due to eccentricity. My opinion 

 all along has been that the temperature of space is little above 

 absolute zero. 



As an argument against the conclusion that space can have 

 the high temperature assigned to it by Pouillet and Herschel, 

 he says: — " Photometry shows that the combined light from 

 all the stars visible in the most powerful telescope is not a 

 millionth of that received from the sun, and there is no reason 



* " Evaporation and Eccentricity as Co-factors in Glacial Periods," 

 Geological Magazine for November 1881. 



