This fact has been demonstrated by experiments : of some samples of ground phosphates, fulfilling in all respects exactly similar conditions, it was found that the sample which had been ground to the finest powder gave 25f per cent. of soluble phos phates ; while it was impossible to get far beyond 22i per cent. with that which had been passed through a " 38-wire " sieve. The material cannot, in fact, be rendered too fine ; and a good plan is to regrind all that does not run through the hand as an impalpable dust, like flour, or the very best Portland cement. It should he caught from the mill in a sieve, and be constantly tested.
Mixing.—The dry materials to be employed in the composi tion of the manure, whether ground mineral phosphate, crushed bones, shoddy, scutch, &c., are raised to the " mixer " by some such apparatus as that shown in Fig. 943. This is a chain elevator, made by Turner, of Ipswich, and is found to answer exceedingly well. The charge of dry materials is shot down at the foot of the elevator, to be raised by the buckets, and deposited by them, in a constant and regular stream, into the hopper with which the top of the mixer is fitted. The ele vator shown in the figure will deliver about 4-5 tons an hour ; but they may be made with 9-in. buckets, to raise 8 tons in the same time. For ground materials alone, which yield readily to the buckets, ordinary pulleys with indiarubber belting may be used, instead of the octagonal pulleys and iron chain ; but the latter repay their extra cost by longer wear.
Fig. 944 shows a front view of the buckets, with a portion of the casing removed. The requisite parts are two octagonal pulleys, cost about 21. ; double wrought-iron chain, about ls. a foot ; plate iron buckets, with steel mouth-pieces, each 28. 9d.; and bolts and nuts for ditto, about ls. 3d. a doz. As the pitch of the chain is 81 in., and every alternate link carries a bucket, there will be one bucket required for every 16} in. of chain, and 2 bolts and nuts to each bucket. The elevator drum should travel at about 20 rev. a minute.
Fig. 945 shows another form of elevator for dry materials, which may be conveniently used when the latter have to be brought from a considerable distance ; but as it is much more expensive than the preceding form, it is only adopted when circumstances compel it. The truck is filled with 4 cwt. of the ground material at the foot of the inclined plane, up which it is drawn by the chain. On reaobing the top, the wheel is caught by a stop on the rail, and the contents of the truck are tipped into the hopper. The truck should make an ascent once in every three minutes.
The "mixer," shown in end view, and longitudinal and cross sections, in Figs. 946, 947, and 948, is also of Turner's construction. It consists of a stout wooden case of 3-in. deal, strengthened by longitudinal tie-rods of x / in. round iron. The boards are all planed, and ploughed and tongued ; the tongues are 1 in. x + in., and are placed in the centre of the thickness of the boards.
The top of the case consists principally of one board 9 in. wide, to which are hung four doors or covers, two on either side. The durability of the case is increased by lining it with " 14-lb." lead, at an extra cost of about 20/. Iron cases have been tried, but have proved less durable than wood. Through the case, passes an octagonal shaft or spindle of cast-iron, into which cast-iron stirrers are wedged helically along its length. The price of such a machine, with feed-roll, hopper, and driving-pulleys, will be about 50/. ; and its weight, say 35 cwt. The driving-strap should be 5 in. wide ; and the speed, 80 rev. a minute. The power required is about 4 The action of the machine may be made continuous or intermittent. In the latter case, it is charged about once in every 3-5 minutes with 4-5 cwt. of ground materials, and the proper propor tion of acid ; it will then turn out about 8-10 tons of manure an hour. By the time that the com pound reaches the exit of the mixer, it should have become very uniformly mixed. The smaller sized mixer will make about 1i-2 tons an hour, and needs little more than 1 H.-P. It is of primary import that the acid used be of one constant degree of strength, or as near thereto as is practicable, and that the quantity used in each charge or mixing be accurately proportioned to the needs of the raw material. Chamber acid, which is ordinarily made at about 115° Tw., is perhaps the best suited for the purpose, as it is sufficiently strong, and is, at the same time, not so strong as to need the addition of water. When the manure-manufacturer also makes his own acid, it may be conve niently run from the chambers ; but as it would be very difficult to regulate minutely the delivery of a certain quantity of acid from so large a vessel as the leaden chamber, it is preferable to let the acid run first into a lead-lined tank, holding about 10 cwt. of acid, placed near the mixer, and at a height of about 4 ft. from the floor. A floating gauge-glass will readily indicate the height of the acid ; and at the side, may be fixed a leaden rule, graduated into divisions, each representing 10 lb. of acid. The tank communicates with the mixer by a 3-in. leaden pipe, fitted with an earthenware tap ; by these means, and by observing the index, the attendant can regulate to a nicety the exact quantity of acid to a given weight of dry material. When the mixing is conducted intermittently, it is well to take care that, after a mixing has been let out, and the exit door has been closed, the flow of acid into the mixer shall be in advance of the dry materials, rather than allow the latter to precede the former, as, in this case, a hard, dry mass may accumulate at the mouth of the mixer, and create much trouble.