72 EFFECTS OF FORESTS ON nUMIDITY. 



day long, it burns the carbon in the soil with great rapidity into 

 carbonic acid ; and this gas, unless there be in the soil some oxide 

 having an affinity to it to retain it, goes off in gas, injuring the 

 salubrity of the air perhaps, and at all events wholly impoverishing 

 the soil, — for carbonic acid is the principal food of all plants. The 

 same course of things it might be shown happens with regard to 

 ammonia ; and thus, both as itself the immediate food of plants and 

 as that which by oxidation yields nitre, ammonia is lost. Thus the 

 indiscriminate destruction of forest over any great breadth of country, 

 if that country has plenty of sunshine, is a great evil." 



In correspondence with Dr Macvicar on the subject, he wrote to me, 

 — " As to the point to which you refer — the rapid generation in the 

 soil, and the vaporization from it of ammonia and carbonic acid, 

 under the impact of the tropical sunbeams and breezes — I do not 

 remember to have seen it dwelt upon by any writer, though both in 

 the writings of Boussingault and of Liebig it is implied ; and it is an 

 inevitable consequence of the eremacausis, or the slow combustion of 

 organic matter. It is familiarly verified by the process of bleaching. 

 Even with such sunshine as we have in this country the organic 

 matter which imparts colour to tissues is carried off much more 

 rapidly in the sunshine than out of it ; and it can only be carried off 

 as carbonic acid and ammonia : that is, it is not merely changed from 

 coloured to colourless, as is proved by the loss of weight which the 

 web has sustained when it is bleached. In this country we have 

 generally less sunshine than we require, and, except in fallows, the 

 surface of the soil is never left bare. Hence the effect of the impact 

 of the sunbeam has been but little considered. But from what I 

 have observed in the tropics, I am persuaded that its power of 

 affecting plant food in the neighbourhood, by the destruction of the 

 fertility of every surface that is left bare, is very great. It intensifies 

 to a wonderful extent that action of the incumbent atmosphere by 

 which the carbon and hydrogen in the soil unite with the oxygen and 

 nitrogen of the air, and give moisture, carbonic acid, and ammonia 

 almost immediately after, if they be not utilized on the spot where 

 they are generated, dispersing all but that quantum which the soil 

 can retain at a temperature of perhaps 140° or 150° F., which is 

 not much." 



