“A cow can furnish each week about 70 liters of milk containing 460 grams of phosphate. This phosphate comes from hay, which obtains it from the soil. But as the soil contains only a moderate quantity of it, and the hay continually takes it away, the supply will at last become exhausted and the milk will become poorer and less abundant. If a kilogram of powdered bones, containing about the same quantity of phosphate as the 70 liters of milk, is spread over the pasture, it will make good the weekly loss in phosphate that the soil undergoes in the production of the cow’s milk. Hence the efficacy of powdered bones on exhausted pasture land. “Phosphoric acid combined with other substances is found in all our agricultural products, and hence the phosphate from bones has a very marked effect on our crops. Harvests have been doubled as if by magic through the use of powdered bones. A kilogram of this powder contains enough phosphoric acid for the growth of a hundred kilograms of wheat. Despite their great value as a fertilizer bones will never be thus used except to a limited extent, because they are not abundant enough and also because they are much in demand in various arts and manufactures. Fortunately in some localities phosphate of lime is found in certain coarse pebbles called nodules or coprolites. These precious stones are carefully collected and ground to powder in a mill. Then, in order to make the substance more soluble in damp soil, and thus better fitted for the nutrition of plants, it is sprayed with an extremely corrosive liquid called sulphuric acid or, more commonly, oil of vitriol. In this way is obtained the superphosphate of lime which manufacture gives to agriculture as one of the most powerful of fertilizers, especially for the raising of grain. “We were wondering a little while ago what substances could be contained in the ashes of a burnt plant, and we have now found potash to be one of them. Moreover, since all vegetation must have phosphate in order to thrive, this also ought to be found in the ashes, phosphate being indestructible by heat. And, in fact, after the incineration of any vegetable matter whatever, as a bundle of hay or a handful of grain, the delicate processes of science can always recover this compound of phosphorus; and they further find lime, iron in the form of rust, the silicious component of pebbles, and divers other substances of less interest. “To finish this difficult but very important subject of the nutrition of plants, I must say a few words about ammonia. This word does not tell you anything since it is a new word to you. But I will make its meaning clear to you by a familiar illustration.