The uppermost stratum of our globe undergoes the direct action of the light and heat of the all-vivifying "orb of day." This action, very unequal in its effects, and most important to understand, has scarcely been touched as yet by scientific research. Our geologists, having been more busily engaged with the inside than the outside of the earth, have broached certain plausible theories--for the most part of a very dubious character--respecting the central fire, Plutonism and Neptunism, the stratification of the planets, the formation of mountains, valleys, and basins. Our mineralogists, thinking far less of the chemical molecular constitution of the different formations than of their crystalline constitution, have minutely studied the physical qualities and geometrical forms of the integral parts of the rocks; but neither have condescended to direct their inquiries to the layer of soil trodden underneath their feet. Yet this very layer of arable earth, to which all bodies must return after death what they have taken from it during life,--this much despised _humus_, furnishes all our agricultural products, the very foundation and support of our material existence. To touch industrial occupations--to meddle with trade, commerce, or agriculture--is unworthy of Science! Such is the silly cry of the many distinguished savants who pride themselves on what they call their "freedom from selfish considerations." Be it so; but then you ought surely to be consistent, and never regard science as a profession or a bread-winner. ON THE CHEMICAL ACTION OF LIGHT. It is no easy study to investigate the modifications and chemical effects which the terrestrial surface is capable of receiving or undergoing, either from the direct rays of the sun, or from diffused light. It requires new methods of inquiry,--methods frequently of extreme delicacy, as the labours of Bunsen and Roscoe, of Kirchhofer and Tyndall, have abundantly demonstrated. Let us here confine ourselves to establishing the fact that the chemical action of light varies according 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 solstices. 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,[78] 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.