With sodium arsenite the dilutions were carried further, to 1/250,000,000, but this still depressed barley to some extent (Fig. 13). With peas the results vary somewhat in the different tests, the depression with 1/2,500,000 and less being usually slight, though occasionally it is much more strongly marked (Fig. 14). In a single series with sodium arsenate barley was apparently unaffected by a concentration of 1/1,000,000, but from this point down to 1/250,000,000 a constant _depression_ showed itself, which was paralleled by a similar depression in the sodium arsenite series from 1/25,000,000 to 1/250,000,000, the curves grading downwards instead of up towards the normal. With peas sodium arsenate has little or no action, though it is just possible that the rather irregular curves indicate a very slight depression below the normal throughout. [Illustration: Fig. 13. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of sodium arsenite and nutrient salts. (Feb. 10th–April 18th, 1913.)] [Illustration: Fig. 14. Curve showing the mean value of the dry weights of ten series of pea plants grown in the presence of sodium arsenite and nutrient salts. (June 27th–Aug. 10th, 1911.)] (_b_) _Toxic effect of arsenic compounds in sand cultures._ Comparatively few tests seem to have been made as to the action of arsenical solutions in sand cultures. Stoklasa (1898) repeated his water culture work, using sand as a medium, and found analogous results by the two methods, i.e. that arsenites are far more toxic than arsenates, and also that the degree of toxicity of a salt varies with the plant to which it is applied, as was shown by the fact that different plants lived for varying times when treated with similar strengths of solution. (_c_) _Toxic effect of arsenic when applied to soil cultures._ Daubeny (1862) watered barley plants with a solution of arsenious acid, 1 ounce in 10 gallons, five times in succession, and found that the crop arrived at maturity about a fortnight earlier than the untreated part of the crop, though the amount harvested was rather less. With turnips four waterings had no effect upon the time of maturity, but again the crop was slightly decreased. The analyses made indicated that no arsenic was taken into the tissues, but that it merely adhered to the external surfaces.