*Action of the Bird-Dung Bate.*--The depleting action of the pigeon- and hen-dung bate is very similar to that of the puer, or dog-dung bate; but the bating process with these materials, as we have seen (p. 18), is carried out at a lower temperature, and is consequently more prolonged. The principal difference between the two bates appears to be a chemical one, due to the fact that bird dung contains all the urinary products which are present only to a small extent in the dung of mammals. In birds uric acid is the chief stage in nitrogenous katabolism, the mechanism of its formation being a process of synthesis in the liver (Halliburton). Urea is also present in considerable amount, and does not appear to be so easily decomposed as the urea in animal urine.[41] As we shall show in the next chapter, urea, and probably also urates, greatly facilitate the permeability of gelatine, and to this fact may be ascribed the more gradual action of bird-dung bates. If we attempt to bate hides with dog-dung, the grain of the hide is found to be attacked and destroyed before the bate has penetrated to the interior of the hide. On the other hand, a bird-dung bate may be used at a temperature of 38° to 40° C. for the puering of skins destined for light leather, but its action is not so favourable as that of the puers. [41] Urea is present to the extent of 5 per cent. in samples of Peruvian guano. COMPOSITION OF BIRD EXCRETA. ------------------------+---------------+-------+-------+------ | Pigeon | Hen | Duck | Goose ------------------------+---------------+-------+-------+------ Moisture | 58·32 | 56·08 | 60·88 | 46·65 | 77·08 Organic matter† | 28·25 | 19·56 | 19·22 | 36·12 | 13·44 Phosphates | 2·69 | 2·54 | 4·47 | 3·15 | 0·89 Carbonate and sulphate} | 1·75 | 3·08 | 7·85 | 3·01 |} of calcium } | | | | |} 2·94 Alkaline salts | 1·99 | 0·82 | 1·09 | 0·32 |} Silica and sand | 7·00 | 17·92 | 6·69 | 10·75 | 5·65 ------------------------+-------+-------+-------+-------+------ |100·00 |100·00 |100·00 |100·00 |100·00 ------------------------+-------+-------+-------+-------+------ † Containing nitrogen } | | | | | equal to ammonia } | 1·75 | 1·21 | 0·74 | 0·85 | 0·67 ------------------------+-------+-------+-------+-------+------ Macadam[42] states that pigeon dung is the most concentrated. Hen manure contains the largest proportion of phosphates, and is followed by duck droppings. That of the goose is the least valuable. The preceding table is taken from his paper. [42] Manures, Natural and Artificial, W. Ivison Macadam. Jour. Soc. Chem. Ind. 1888, p. 79. Procter[43] quotes the following, as a mean of 40 analyses of pigeon dung by Schulze:-- [43] Principles of Leather Manufacture, p. 179.