The observations which have been made regarding the use of these substances, lead directly to the inference that all vegetable matters possess a certain manurial value, and that they ought to be carefully collected and preserved. In fact, the careful farmer adds everything of the sort to his manure heap, where, by undergoing fermentation along with the manure, their nitrogen becomes immediately available to the plant; while the seeds of weeds are destroyed during the fermentation, and the risk of the land being rendered dirty by their springing up when the manure comes to be used is prevented. FOOTNOTES: [Footnote M: The presence of sulphuret of sodium in this case is due to the difficulty of completely burning the ash. It exists in the plant as sulphate of soda.] CHAPTER X. COMPOSITION AND PROPERTIES OF ANIMAL MANURES. Manures of animal origin are generally characterized by the large quantity of nitrogen they contain, which causes them to undergo decomposition with great rapidity, and to yield the greater part of their valuable matters to the crop to which they are applied. _Guano._--By far the most important animal manure is guano, which is composed of the solid excrements of carnivorous birds in a more or less completely decomposed state, and is accumulated in immense quantities on the coasts of South America and other tropical countries. It has been used as a manure in Peru from time immemorial, but the accounts given by the older travellers of its marvellous effects were considered to be fabulous, until Humboldt, from personal observation, confirmed their statements. It was first imported into this country in 1840, in which year a few barrels of it were brought home; and from that time its importation rapidly increased. Soon after large deposits of it were found in Ichaboe; and it has since been brought from many other localities. The quantity of guanos of all kinds imported into this country and retained for home consumption now exceeds 240,000 tons a year. The value of guano differs greatly according to the extent to which its decomposition has gone, and this is chiefly dependent on the climate of the locality from which it is obtained. When deposited in the rainless districts of Peru it still retains some of the uric acid and the greater part of the ammonia naturally existing in it, and the quantity which has escaped by decomposition is unimportant. But that obtained from other districts has suffered a more or less complete decomposition according to the humidity of the climate, which reduces the quantity of organic matters and ammonia, until, in some varieties, they are so small as to be of little importance. The following are minute analyses of three specimens of Peruvian guano, shewing all the different constituents it contains, and the amount of difference which may exist:--