232 



pbotograpLs thus obtained the accompanying illustrations were made. 

 In these experiments, for the sake of comparison, one pot of soil was 

 prepared without addition of any poisonous compound. Now, it is quite 

 evident from these plates that the arsenical compounds in the soil did 

 l)roduce an injurious effect, and in some instances, in fact in the 

 majority of them, it was decidedly marked. In case of the Paris-green, 

 as shown in Plate 1, it is not noticed until the quantity present iji the 

 soil reaches 500 milligrams, and that in the other pots the size of the 

 plants decreases regularly as the quantity of arsenical compound present 

 increases. The numbers on the pots in the different plates represent 

 the quantities present in grams and tenths of grams. 



In case of the arsenite of potassa, (Plate 2,) the effect seems to be 

 more immediate. This may be due either to the greater solubility of the 

 compound or to a possibly larger quantity of arsenious acid. The 

 potash compound seemed nearly pure, being crystallized, and the purity 

 of the copper compound was not estimated. Yet in this case the effect 

 does not seem decidedly marked until the quantity present reached 300 

 milligrams. 



To the influence of arseniate of potassa the plants seemed to be more 

 sensitive still, for those in the pot containing but 100 milligrams 

 seem to be affected. Yet even when 200 milligrams are present, the 

 plants seem to thrive tolerably well. 



What, then, are the quantities of these compounds which may be ap- 

 plied to the soil for the various purposes in practical agriculture before 

 effecting any injurious results'? The amount of soil in each of the pots 

 employed in these experiments was 91.5 cubic inches. In case of the 

 Paris-green, the limit is 500 milligrams for this quantity of soil, which 

 is equivalent to 145.6 grams per cubic foot, or 906.4 pounds per acre, 

 calculating for a depth of one foot. The limit for arsenite of potassa, 

 being 300 milligrams per 91.5 cubic inches, is about 540 pounds per 

 acre. Though the plants seem to be affected by even a small quantity 

 of arseniate of potash in the soil, I am nevertheless inclined to the opin- 

 ion that this compound may be aiiplied at the rate of 150 j)ounds per acre 

 without any great injury to the crop. For practical purposes, however, 

 it is never necessary to apply in any case so large an amount. 

 . These results are confirmed by the water-culture experiments of Pro- 

 fessor Prey tag * in his investigations upon the influence of the sulphur- 

 ous and metallic fumes of the Freiberg Metallurgical Works upon the 

 vegetation of the surrounding fields. He found that plants were killed 

 when j)laced in solutions containing gL jier cent, arsenious acid, ^ 

 per cent, sulphate of zinc, -J^ per cent, sulphate of copper, Jg- per 

 cent, sulphate of cobalt, jL per cent, sulphate of nickel, and i per cent, 

 sulphate of iron. 



Mons. E. Heckel t states that 25 milligran s of arsenious acid or 

 the soluble arseniates in 90 grams of water prevents germination and 

 destroys the embryos of seeds. 



I cannot, however, agree with Professor Freytag in the statement 

 that the arsenious acid and the oxides of zinc and lead cannot be injuri- 

 ous to vegetation on account of their property of forming insoluble com- 

 pounds in the soil, since in some of my preliminary experiments pres- 

 ence of such insoluble compounds as the arseniates of barium, strontium, 

 and magnesium was sufficient to prevent germination. Again, in re- 

 ports upon the composition of certain mineral-waters of Germany, we 



* Jtihrbuch fur Berg- und Hiittenvvesen, 1873. 

 t Coniptcs reudus, t. LXXX, 1172. 



