24 Effect of Copper Compounds 



quantities of solids causes progressive dilution of the toxic medium, 

 the underlying cause of these results being the gradual removal of 

 molecules or ions from the solutions by the insoluble body present. 



Fitch's results are also in accordance with the well-known fact that 

 the physical condition and properties of the added solid play a consider- 

 able part in determining its efficacy as an adsorbing agent. 



(d) Effect of copper on plant growth when present in soils. 



As has already been shown the toxic property of copper with regard 

 to plants was recognised almost as soon as that element was found to 

 occur in the vegetable kingdom, but little notice was taken of the 

 discovery for many years. In 1882 F. C. Phillips asserted, as the 

 result of experiments with various cultivated flowering plants, including 

 geraniums, coleas, ageratum, pansies, &c., that under favourable condi- 

 tions plants will absorb small quantities of copper by their roots, and 

 that such compounds exercise a distinctly retarding influence even if in 

 very small amount, while if large quantities are present they tend to 

 check root formation, either killing the plants outright or so far reducing 

 their vitality as seriously to interfere with nutrition and growth. Two 

 years later Knop confirmed both the absorption and the toxicity of 

 copper by his experiments on maize. 



Jensen (1907) worked with " artificial " soils, under sterile conditions, 

 using finely ground quartz flour for his medium and wheat for a test 

 plant, parallel experiments being carried on with solutions. Every 

 precaution was taken to ensure sterility the corks were boiled first in 

 water and then in paraffin, the seeds were sterilised in 2 / copper 

 sulphate solution for f hour, washed in sterilised water, planted in 

 sterilised sphagnum, the transplanting being done in a sterile chamber 

 into sterilised solutions. The criteria used to determine the toxic and 

 stimulation effects were the total transpiration, average length of sprout, 

 the green weight and dry weight of plants. The results obtained with 

 the different substrata showed that it does not follow that a salt highly 

 toxic in solution is equally so in soil, or that one which holds a relatively 

 high toxic position in soil should occupy the same relative position in 

 solution cultures. For instance, while in soil cultures nickel compounds 

 were the most toxic of all the substances tried, in solution cultures 

 silver compounds were more poisonous than nickel. The range of con- 

 centrations, both fatal and accelerating, was found to be much greater 

 in solution than in soil cultures. 



In the sand cultures the toxicity of the copper sulphate was found 



