In order to make our survey of the composition of urine uniform with that of the dung, let us see how the urine of the common farm animals compares in the matter of the composition of its dry substance. The following results (basing our calculations on Stoeckhardt's figures, previously given) show this:-- Nitrogen, Phosphoric acid, Alkalies, per cent. per cent. per cent. Pig 12.0 5 8 Horse 10.9 trace 13.6 Sheep 10.4 3.7 14.9 Cow 10.0 trace 17.5 From these figures we see that the dry substance of the urine of the pig is richest in nitrogen and phosphoric acid, but poorest in alkalies, of the four common farm animals; that of the horse comes next in the amount of nitrogen it contains, but that, on the whole, there is very little difference between the horse, cow, and sheep in this respect.[136] As in the case of the dung, this subject is best studied in relation to the food consumed. We are again indebted to Wolff's investigations for valuable information on this point. He has found that the following percentages of _organic matter_, _nitrogen_, and _mineral substances_, originally present in the dry matter of the food, are voided in the urine:-- Cow. Ox. Sheep. Horse. Average Organic matter 4.0 4.4 2.0 3.3 3.4 Nitrogen 31.0 54.8 42.3 60.7 47.2 Mineral substances 43.1 34.3 41.0 37.5 39.0[137] We have now considered briefly the composition of the solid excrements and urine of the common farm animals, and have also enumerated some of the principal causes of the variation in their composition.