206 ANNUAL OF SCIENTIFIC DISCOVERY. 



1. Way, and, after him, Liebig, have shown that every soil absorbs ammo- 

 nia, and also potash, from solutions containing them or their salts, generally 

 leaving the acid, which takes up lime, etc., from the soil in solution. The 

 ammonia and potash, which are absorbed in very large proportion by arable 

 soils, are rendered thereby quite insoluble. 



2. Arable soils absorb also silicic acid in very considerable proportion, 

 and it also becomes insoluble. 



3. Arable soils also absorb the phosphoric acid of phosphate of lime, or 

 of ammoniaco-magnesian phosphate, apparently soluting the acid, which 

 also becomes insoluble. 



4. Hence the soluble ingredients of manures cannot be conveyed to the 

 plants in the form of a solution percolating the soil (such as liquid manure, 

 or a solution formed by rain-water with the aid of carbonic acid), since such 

 a solution is deprived of its dissolved ingredients by filtering through a 

 very moderate amount of soil. 



5. Hence, also, as the food of plants must thus be fixed in the soil in an 

 insoluble form, it is plain that it can only enter the plant in virtue of some 

 power or agency in the roots, which decomposes the insoluble compounds 

 in the soil, and thus renders soluble the necessary matter. 



6. The absorbent power of soils is partly chemical, and partly mechanical, 

 as is the case with charcoal. 







7. The quantities of alkalies, of phosphates of ammonia, etc., capable of 

 being supplied to plants by rain-water, after it has been percolated through 

 the soil, even supposing the whole to be assimilated, does not amount to 

 more than a mere fraction of what the plants contain. 



8 The theory of the transference of ammonia, potash, silica, phosphates, 

 etc., from the soil to the plant, is not yet understood; but the old theory, 

 that the rain conveys food to the plant directly, is certainly not the true one. 

 Edin. New Phil. Journal. 



ON THE PRESENCE OF ARSENIC IN PLANTS USED FOR FOOD. 



The following is the substance of a paper on the above subject, recently 

 read before the Natural History Society. Dublin, by Prof. John Dav3 r : 



His attention was first attracted to the subject by the difficulty of obtain- 

 ing pure sulphuric acid, that sold in shops commonly containing more or less 

 arsenic, derived from the pyrites from which it is manufactured. Super- 

 phosphate of lime is extensively used as a manure, and is more and more 

 employed every day, and this manure is made by the addition of sulphuric 

 acid to crushed bones ; and, in order to produce it at the lowest cost, an infe- 

 rior description of acid is used, containing, among other impurities, an in- 

 creased quantity of arsenious acid or white arsenic. With a view to ascer- 

 tain whether plants had the power of absorbing this arsenic from the earth, 

 Professor Davy transplanted three small pea-plants, and when they had 

 recovered from the removal, he watered them every other day for about a 

 week. The plants did not appear to be injured by the treatment, but grew 

 up, flowered, and produced seed as usual. Having collected the stalks, 

 leaves, and pods, they were carefully put aside for examination. The means 

 most commonly employed for the detection of minute quantities of arsenic 

 are Reinsch's and Marsh's. On trying the plants by these tests united, it 

 was found that not only the stalks and leaves, but even the seeds had 

 absorbed the poison, which was thus found to have penetrated the entire 

 plant. 



