Further corroboration of True and Oglevee’s work was obtained by Fitch (1906) who worked in a similar way with fungi, arriving at the general conclusion that insoluble substances in a solution act as agents of dilution or absorption whereby poisonous ions or molecules are in some way removed. He found that _n_/256 of copper sulphate in beet concoction exercised a stimulating effect on _Penicillium glaucum_, but the addition of fine glass to the solution increased the stimulation, while large or medium sized pieces did not have the same effect. This action of solid bodies in reducing the deleterious effects of poisonous solutions is attributed to the process of “adsorption” whereby a layer of greater molecular density is formed on the surfaces of solids immersed in solutions. The solids presumably withdraw a certain proportion of poisonous ions or molecules from the body of the solution (retaining them in a molecularly denser layer over their own surfaces), so that the toxic properties of the solution are reduced owing to the withdrawal of part of the poison from the field of action. In some cases this reduction may be so great as to relieve the solution of its toxic properties, or even to cause an abnormal acceleration to replace a marked retardation. Also, if the solution is of such a dilution as to cause acceleration of growth in plants, the addition of insoluble substances may increase this acceleration. The progressive addition of 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 considerable 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 conditions 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.