290 Brenchley.—On the Action of Certain Compounds of 
Arsenic Acid. 
Arsenic acid is far less toxic than arsenious acid in its action on 
barley. Concentrations ranging from 1/100,000 to 1/50,000,000 arsenic 
acid gave a series of plants in which no definite toxicity was manifested, 
though the shoot growth with the highest strength (1/100,000 As 2 0 5 ) indi- 
cated a slight depression, which, however, was not evidenced by the roots 
(Curve 8). The whole series of plants were of a particularly dark green 
colour, which contrasted very strongly with the yellowish green tinge 
shown by a series of plants of the same age ranged alongside, which were 
growing in the presence of zinc sulphate. The general appearance of the 
gm. 
Curve 8. Showing the mean values of the dry weights of ten series of barley plants (‘English 
Archer’) grown in the presence of arsenic acid and nutrient salts. (Feb. 28-April 24, 1911.) 
plants with arsenic acid was very favourable. Higher strengths of the 
poison have not yet been tested, so the degree of toxicity of arsenic acid for 
barley is uncertain. 
Sodium Arsenite. 
Barley. The highest strength of sodium arsenite which admits of any 
appreciable growth in barley is 1/100,000, but the plants are very weak 
indeed, and are liable to succumb at the least provocation. With decreasing 
concentration rapid improvement in growth is noticed, which is specially 
well marked in plants grown in the summer (May-July). In spite of this, 
however, a marked depression continues evident even with as little poison 
as 1/50,000,000 (Curve 9). Although the dry weights indicate this con- 
tinued toxic action, the appearance of the plants points to a possible 
stimulus with concentrations of 1/5,000,000 and less. This depression was 
not completely eliminated even when the strength of the poison was 
