mere matter of adaptation and adjustment, since toxic substances are not usually present in soil, but this assertion is too sweeping to be accepted in its entirety, although it probably holds good to a certain extent with some species of plants. Kahlenberg and True (1896) found that the addition of an organic substance produced the same effect as the addition of some nutrient salt, in that it reduced the toxicity of the copper salt, e.g. in the presence of sugar and potassium hydrate the lupins were able to withstand a concentration of 1/400 copper sulphate, part of which reduction of toxicity is attributed to the sugar. (_c_) _Effect of adding insoluble substances to solutions of copper salts._ Other investigators have shown that the presence of insoluble substances has a similar effect in reducing toxicity to an even greater degree. True and Oglevee (1904, 1905) again used _Lupinus albus_ as a test plant in the presence of solutions of various poisons in pure distilled water, copper sulphate, silver nitrate, mercuric chloride, hydrochloric acid, sodium hydroxide, thymol and resorcinol all coming under consideration. Clean sea sand, powdered Bohemian glass, shredded filter paper, finely divided paraffin wax and pure unruptured starch grains were respectively added to the solutions, and seedlings were suspended over glass rods so that their roots were in the solutions for 24–48 hours. The solids varied in their action on the different poisons; while the toxic influence of mercuric chloride was reduced by sand and crushed glass, the action of silver nitrate was modified by nearly all the solids. Lupin roots proved unable to withstand an exposure of 24 hours to a concentration of copper sulphate of 1 molecular weight in 60,000 litres of water (i.e. about 1 part by weight CuSO₄.5H₂O in 240·4 parts water), but the addition of solids caused a great decrease in toxicity. When the amount of copper was diminished an advantage was regularly obtained in favour of the cultures containing the solid bodies. On the whole the ameliorating action of solids is more clearly marked with dilute solutions of strong poisons than with relatively concentrated solutions of weaker poisons. As a general rule, filter paper and potato starch grains exert a more marked modifying action than the denser bodies, such as sand, glass or paraffin. Breazeale (1906) tested the same point with extracts of certain soils which proved toxic to wheat seedlings grown in them as water cultures. The toxicity was wholly or partly removed by the addition of such substances as carbon black, calcium carbonate or ferric hydrate. Other experiments showed that the toxic substances of ordinary distilled water are removed by ferric hydrate and carbon black, and further that the latter substance will take out copper from copper solutions, rendering them far less poisonous.