Fig. 931 shows what is known as a " dust bone-mill," consisting of a strong cast-iron frame, and two rollers furnished with steel cutters or saws, through which the bones, already ground by the mill just described, are passed, and thus reduced to dust. It requires 6-H.P.; weighs 26 cwt. ; works at a speed of 240 rev. a minute ; turns out 6 tons daily ; and costs about 100/.
Occasionally, the ground bones are dissolved in sulphuric acid before nse. This no doubt renders the phosphates more readily soluble, and available for the plant ; but there is so much difficulty in drying the bones afterwards, that a large amount of free sulphuric acid exists in tho mass, and destroys the sacks in which the manure is transported. There is also inconvenieace in dtilling-in the material with the seed, as is often done with dry manures. As an ingredient in other manures, however, e. g. " bone-superphosphates," &e., bones play an important part, and their presence in a superphosphate adds greatly to its value.
Fish.—Fish and fishery-offal are valuable fertilizers, rich both in phosphates and in nitrogen. In the Eastern Counties, and some other spots round the British coasts, they are sometimes applied in a raw state, when it is impossible to find a ready market for them as Inman food. They are often treated with sulphuric acid, in large leaden tanks, after having been pressed to extract the oil, which is, in itself, a valuable product, and whose presence would greatly neutralize the effect of the acid. In this country, very little care is bestowed on the preparation of fish-manures, and their quality varies suspiciously ; but the Norwegians manufacture an excellent fish-guano, containing 25-30 per cent. of phosphate of lime, and over 7 per cent. of nitrogen. The guano-factories at Lerosen, Sanoen, and Lyngvoor produced 23,650 sacks (of 2 cwt.) of fish-guano in 1875, 23,061 in 1876, 22,561 in 1877, and 21,860 in 1878.
The Americans are awaking to the value of fish-manures, and on many parts of the eastern coast of the United States, factories have been erected for the utilization of the shoals of menhaden, a fish of the herring family, which frequent those shores between April and November. In 1873, there were 62 factories at work on the coasts of New York and New England, catching at the rate of 1,193,100 barrels of fish, yielding 2,214,800 gal. of oil, and 36,299 tons of guano. Since then, the industry has much increased, particularly in N.-E. Long Island. The first step after catching the fish is the expression of its oil, which process will he described under Oils. The oil and moisture
having been removed, the refuse fish is taken to the " scrap-house," and is known as " green scrap." In 24-48 hours, fermentation sets in, producing a darker shade, by the escape of ammonia ; the material is then called " old scrap." In this state, it is transferred to a drying-room, where it is first subjected to a " picking " process, which consists iu passing it through a cylinder armed with teeth revolving between set teeth, by which the whole mass is rendered uniformly fine. It is then dried, either in the sun, or by artificial heat. By the former plan, it is spread upon a sloping " platform," and constantly stirred by a wooden barrow, being finally gathered into a large heap, called the " cure," into which perforated pipes are inserted for conducting away any heat that may be developed. After about 4 turnings, it is cool enough for subsequent treatment. In wet weather, "platform curing" is replaced by artificial heat, which is a quicker process, but causes 10 per cent. more loss. The driers are revolving cylinders, with shelves running spirally through them. A fire is made at the front end ; the hot air from this passes beneath the cylinder to the back end, and returns through the cylinder to the chimney. The drier is fed in front, and as it revolves, the scrap is carried up by the shelves to the top, whence it falls, to be taken up again in the same way. The archimedean arrangement of the shelves gradually works the material to the outlet of the cylinder. In a 25-ft. cylinder, revolving 8 times a minute, each charge takes about hour to reach the back end, by which time its moisture will have been removed, and the material made ready for the "cure." If very wet, it may require 2-5 dryings. It is evident that the finest particles of the material will be carried off by the draught. Green scrap is mostly used for platform drying ; old scrap, if very wet, is spread on the platform for 12-24 hours, before being put through the driers, or it would cake into balls. The dried scrap is ground and bolted in a special mill, consisting of two cylinders with cone-shaped bearing-faces, one making 2500 rev. a minute, the other 800. Analyses of dried fish-remains show 14 per cent. of phosphate of lime and magnesia, and 12 per cent. of nitrogen. Quantities of fish-" mare," or dried fish-refuse, are imported into this country for manu facture into nitrogenous manures.