CHAPTER VII
PHOSPHATES AND NITROGEN
“Phosphorus, which is a dangerous poison, as we have seen, is nevertheless found in abundance in the bodies of all animals. It occurs in the urine, whence Brandt was the first to extract it; it is found still more plentifully in the bones, and from thence it is now obtained. There is some in meat, in milk, and in cheese; also in plants, notably cereals; hence flour and bread contain it. But do not be alarmed: we shall not die of poison like the rats that have nibbled crusts smeared with grease and phosphorus.”
“But why not,” asked Emile, “if we eat it as the rats do?”
“I will try to explain,” replied his uncle. “When two or more substances are mixed together, they lose their original properties, while the new substance obtained by their combination is found to possess new properties having nothing in common with the old ones. Thus carbon, when combined with the air that we breathe, becomes an invisible gas, subtle, and unfit for breathing. In like manner lime, burning to the taste, is converted by union with carbonic acid gas into chalk, a calcareous stone void of taste. Furthermore, poisonous substances, deadly in a very [[36]]small dose, may become harmless and even enter into the composition of our food when they are combined with other substances. Thus it is with phosphorus. What, then, is united with phosphorus in the form in which it ceases to be poisonous and enters into the composition of meat and flour? That is what we will now consider.
“When phosphorus is burned it produces a thick white smoke, of which you can get some idea by striking a number of matches all at once. This white smoke with the slightest trace of humidity is reducible to an extraordinarily acid liquid called phosphoric acid. Since this compound results from the combustion of phosphorus, just as carbonic acid is the result of the combustion of carbon, it must and in fact does contain the air without which no combustion can take place. Phosphoric acid is no longer inflammable, however much it may be heated; being itself the product of combustion, it cannot burn again. But if there is no danger of its catching fire, phosphoric acid is nevertheless dangerous on account of its intense acidity, which makes it violently corrosive in its action on flesh. If mixed with lime, however, this formidable compound loses its injurious properties and is changed into a white substance without the least taste or the slightest poisonous effect. This substance is called phosphate of lime. Burnt phosphorus and lime, thus united, furnish the greater part of the mineral matter found in bones. Put a bone into the fire: the grease and juices that permeate its substance will be burnt up and the bone will [[37]]lose a part of its weight and become friable and perfectly white. Well, this bone, calcined in the fire for a long time, is composed chiefly of phosphate of lime. It contains phosphorus, the most combustible of substances, and yet is itself absolutely incombustible; it contains one of the most poisonous substances, and yet is itself quite harmless; into its composition there has entered an ingredient possessing atrocious acidity, and yet the compound itself has no taste. Similarly combined and equally harmless, phosphorus is found in meat, milk, cereals, in flour and bread.
“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 [[38]]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. [[39]]
“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.