In colour it varies from a very light to a very dark brown, the richer samples being generally lighter. Samples taken from even the same deposit have been found to differ very considerably in appearance, those taken from the lower and older layers being usually darker than those taken from the more recent upper layers. It was soon found also to vary very much in composition. After a deposit had been worked for some time, the quality of guano it yielded was found to be inferior and coarser, and in many cases mixed with pebbles or pieces of granite, porphyry, &c. This led to the custom of screening it on arrival in this country, before it was used as a manure. In the richer qualities--_e.g._, in the Chincha guano--little round concretionary nodules, varying in colour from pure white to dark brown, were occasionally found. Analysis showed these nodules[190] to be composed chiefly of potash salts. Sometimes, also, little crystals of almost pure ammonia salts were found. It soon became customary, therefore, to prepare guano for the market by separating the stones and reducing the whole to a fine uniform powder. One of its most characteristic properties, and the one which seems to have impressed the public most, was its pungent odour. Undue importance was attached to this property, in the belief that it was caused by the ammonia it contained. It may be doubted, however, whether the characteristic smell of guano is due so much to its ammonia as to certain fatty acids. _Composition._ In composition it is of a most complex nature. It contains its nitrogen in a great variety of forms, the chief of these being urate, oxalate, ulmate, humate, sulphate, phosphate, carbonate, and muriate of ammonia; and also in a rare form of organic nitrogen peculiar to guano, called guanine. According to Boussingault, some guanos contain small quantities of nitrates. Its phosphoric acid is present both in the soluble state--viz., as phosphates of the alkalies (ammonia and potash)--and in the insoluble state as phosphate of lime; and lastly, its potash is present as sulphate and phosphate. The proportion in which these different forms of nitrogen and phosphoric acid are present varies considerably in different samples. The richer a sample, as a rule, the more nitrogen in the form of uric acid it contains. The most of the nitrogen is present as uric acid and ammonia. Damp guanos contain more of their nitrogen as ammonia than dry ones, this being due to the fermentation which goes on in the former. On an average, about a third of its total nitrogen is soluble in water. Of its phosphates, on the other hand, only about a fourth are soluble in water. The following analyses of a sample of Chincha island guano by Karmrodt[191] will illustrate this. (Sample dried at 212 deg. Fahr.):-- 1. _Constituents easily soluble in Water._