ACTION OF HEAT. 313 



Bunsen and Roscoe, of Kirchhofer and Tyndall, have abun- 

 dantly demonstrated. Let us here confine ourselves to estab- 

 lishing the fact that the chemical action of light varies accord- 

 ing to the geological constitution of the soil, according to the 

 diurnal and annual obliquity of the solar rays, according to the 

 hours of the day, according to the latitudes and seasons. The 

 maximum of effects is manifested about the times of the sol- 

 stices. 



For the better co-ordination of these phenomena, might 

 we not, as has been done in regard to the distribution of 

 heat over the terrestrial surface,* link together by lines the 

 points of equality? We should thus create an aggregate of 

 iso-photo-chemical lines, diurnal, mensual, and annual, of 

 incontestible utility for the progress of general physics and 

 meteorology, which are still in their infancy. 



But to realise this magnificent programme, the union and 

 agreement is necessary of scientific men in every region of 

 the globe ; an ideal, therefore, as yet, is very far from being 

 attained. 



THE ACTION OF HEAT. 



The earth is subject to the influence of two different 

 sources of heat. One, like the arterial blood, strikes from 

 the centre to the circumference : this internal heat it is 

 which has been stored up since the unknown epoch when 

 our globe was nothing more than an incandescent nucleus, 

 surrounded by condensable vapours. The other, like the 



* Reference is here invited to Humboldt's thermal and isothermal lines. 

 See Dr A. K. Johnston's "Physical Atlas," and Humboldt's " Kosmos." 



