90 Recent Publications on Manures. 



be carried off by the water, but that the soil decomposes them, 

 and retains the humus, which is insoluble, till acted on again by 

 ammonia, and rendered soluble. If the roots are near, when in 

 this state, it is absorbed; if not, it is decomposed again. These 

 calculations, he says, place in a favourable light the great che- 

 mical value of ammonia in soil, and likewise the importance of 

 humus in the process of vegetable nutrition. 



' On the opinion that carbonic acid is the only source of 

 carbon to the plant, he experimented, by passing a stream of 

 carbonic acid, dried by traversing a tube filled with fragments 

 of chloride of calcium, over 2072 grains of soil, and found no 

 increase of weight, with a balance indicating as low as ~ of a 

 grain. The soil could not, therefore," he says, " have absorbed 

 4 J q per cent of carbonic acid ; and the free carbonic acid in the 

 soil must, therefore, exist chiefly in the form of solution in 

 water; though it cannot be said soil does notabsorb that gas at 

 all, as small imperceptible quantities may exist. The saline 

 manures, l^cwt. per acre, exhibited with great effect, amounting 

 to less than jq^q o P art J or l ess than T ^ per cent, of the soil, at 

 6 in. deep." He preferred using the pure gases in his analysis ; 

 as to have dried the impure from the dung-heap, by caustic 

 potash, would have absorbed the carbonic acid ; or to have 

 employed the chloride of calcium, would have lost the arnmo- 

 niacal vapours by absorption ; and to have used them moist 

 would have altered the result. He next notices the opinion, 

 that humus, if absorbed, will be returned as humus, without 

 being assimilated ; the same as found in sugar and gum by 

 Meyen and others. To test this opinion, he took a plant of 

 common groundsel (Senecio vulgaris), washed the roots clean, 

 and divided them into two bundles. He introduced the one 

 bunch of washed roots into a glass of water containing a small 

 quantity of ulmate of ammonia in solution ; the other half he 

 inserted into another glass vessel containing pure water. As 

 the plant grew, he filled up with pure water. At the end of 

 seven days, when the solution of ulmate of ammonia had become 

 much lighter, evidently by absorption, he took the one contain- 

 ing pure water, which should have now contained humus, if 

 again excreted ; but, on evaporation, he found it to contain only 

 a highly crystalline substance, of a light brown colour, more 

 resembling gum than humus. Macaire also found, he says, that 

 beans excreted a substance resembling gum : and, as he has 

 proved that ulmate of ammonia must always exist in fertile soil, 

 and as he has found plants cannot refuse anything offered to 

 them in solution, having himself poisoned plants by making 

 them grow in solutions of lead ; therefore, beans must have ab- 

 sorbed ulmate of ammonia, and assimilated, not excreted, it. 

 If we can prove, he continues, with one or two plants, that 



