Some years before True and Gies published their results, Coupin (1898) had grown wheat seedlings in culture solutions with the addition of copper salts for several days in order to find the fatal concentrations of the different compounds. Taking toxic equivalent as meaning “the minimum weight of salt, which, dissolved in 100 parts of water, kills the seedling,” the results were as follows: Toxic Containing equivalent copper Copper bromide (CuBr₂) ·004875 ·001387 Copper chloride (CuCl₂.2aq.) ·005000 ·001865 Copper sulphate (CuSO₄.5aq.) ·005555 ·001415 Copper acetate (Cu{C₂H₃O₂}₂.aq.) ·005714 ·001820 Copper nitrate (Cu{NO₃}₂.6aq.) ·006102 ·001312 These numbers appear to be very close, so Coupin considered that it might be permissible to regard the differences as due to the impurities in the salts, and to the water of crystallisation which may falsify the weights, so that under these conditions one may believe that all these salts have the same toxicity. This is considerable, and is evidently due to the copper ion, the electro-negative ion not intervening with such a feeble dose. A recalculation of these toxic equivalents to determine the actual amount of copper present in each, gives results that are fairly approximate, but it is difficult to accept this hypothesis in view of other work in which different salts of the same poison are proved to differ greatly in their action on plant growth. Kahlenberg and True (1896), working with _Lupinus albus_, found that the various copper salts, as sulphate, chloride and acetate, were similar in their action upon the roots. Plants placed in solutions of these salts of varying strengths for 15–24 hours showed that in each case 1/25,600 gram molecule killed the root, while with 1/51,200 gram molecule the root was just alive. These workers discuss their results from the standpoint of electrolytic dissociation, and concur in the opinion that the positive ions of the toxic salt are exceedingly poisonous. The toxicity of the positive ion was again set forth by Copeland and Kahlenberg (1900). Their water culture experiments were carried on in glass vessels coated internally with paraffin to avoid solution of glass, and in tests with seedlings of maize, lupins, oats and soy beans it was found that such metals as copper, iron, zinc and arsenic were almost always fatal to the growth of plants. As a general rule those metals whose salts are toxic, themselves poison plants when they are present in water. The assumption made was that the injury to plants when cultivated in the presence of pure metals depends on the tendency of the metal to go into solution as a component of chemical compounds and on the specific toxicity of the metallic ion when in solution. (_b_) _Masking effect caused by addition of soluble substances to solutions of copper salts._