The solid excreta consist, as we have seen, of _undigested_ food, while the urine contains the manurial ingredients of the food which have been _digested_ by the animal system.[138] The latter is, weight for weight, as a rule, very much more valuable as a manure than the former. From the table given in the Appendix[139] it will be seen that the proportions of the nitrogen and ash-constituents originally present in the food consumed, which are voided in the excrements, vary with different circumstances. Wolff, in summarising his results, points out that, as a rule, the solid and liquid excrements will contain about 46 per cent of the organic matter, 87.3 of the nitrogen, and 98.7 of mineral matter; while the experiments of Lawes and Gilbert at Rothamsted show that, with fattening oxen and sheep and with horses, more than 95 per cent of the nitrogen and 96 per cent or more of the ash-constituents are voided in the manure. The pig retains a larger proportion of the nitrogen--about 85 per cent appearing in the manure--while in the milking-cow only about 75 per cent is returned in the excrements. Generally speaking, we may say that the nitrogen originally present in the food suffers very little loss in passing through the animal system, and that, practically speaking, the ash-constituents suffer no loss whatever. As to the distribution of the manurial ingredients, much will depend on the nature of the food. Almost invariably more than a _half_ of the total nitrogen excreted will be found in the urine, in many cases very much more.[140] Of the mineral constituents, about a third on the average may be said to be excreted in the urine. Of this mineral matter it may be noted that nearly all the alkalies (potash and soda), or about 98 per cent, are found in the urine. Of phosphoric acid and lime, on the other hand, there are the merest traces in the urine. Horse-urine, however, is an exception with regard to lime, as it contains about 60 per cent of the lime consumed in the food. For information on the subject of pig-manure the reader is referred to Appendix, Note V.[141] Before passing from this part of the subject, it may be desirable to place before our readers the composition of the dung and urine taken together, so that we may be able to form some idea of their relative value, weight for weight. As the nitrogen constitutes by far the most valuable portion of the manurial ingredients, it will be sufficient if we compare them as to their percentage of this ingredient. Water, Nitrogen, Calculated on per cent. per cent. dry substance, Analyses by per cent. Sheep 67 .91 2.7 Juergensen. Horse 76 .65 2.7 Boussingault. Pig 82 .61 3.4 Boussingault. Cow 86 .36 2.6 Boussingault.