Figs. 949 and 950 show a complete arrangement of manure-making apparatus :—a is the mixer, which is supplied with dry stuff by the elevators b, and with sulphuric acid by the pipe c (having a plug at d) from the cistern e, which is filled from the chamber by the pipe f ; g is the pit or "den," into which the manure is delivered by the mixer ; and la is the platform for the man who regulates the supply of acid, and opens and closes the delivery-hole of the mixer, by means of the sliding door, worked by a lever, as shown in Fig. 946.
The following are practical notes on the quantity of concentrated sulphuric acid (say 168° Tw.) required by different kinds of phosphatic materials :-1 ton Cambridge coprolites, 60 per cent. phos phate, requires about 14i cwt. ; 1 ton Suffolk coprolites, 50 per cent. phosphate, 15 cwt. ; 1 ton Spanish phosphorite, 68-70 per cent. phosphate, 14 cwt. ; 1 ton Navassa phosphate, 73 per cent. phosphate, 141 1 ton of a mixture of two parts Cambridge coprolites, and 1 part bone, 14 cwt. ; 1 ton of a mixture of 2 parts bone-ash (70 per cent.), and 1 part fin. bone and bone-dust, 13 cwt. When " chamber acid " is used instead of oil of vitriol, an increased quantity of acid, varying in inverse proportion to its strength, will be required. The following table shows the number of pounds of chamber acid, according to its density, required for 1 ton of Cambridge coprolites ; opposite the quantities, are stated the corresponding depths of acid in inches and tenths, as measured from a cistern having the dimensions 6 ft. x 4 ft. :— The effect of the chemical action of the acid on the phosphate is the generation of considerable heat, which is of great service in rendering the manufactured article thoroughly dry, so that it can be reduced to a moderately fine powder. For the purpose of conserving the heat, it is common to make the mixer-pit of very large size, capable of holding as much as 100 tons at one time ; and it is found that much greater proportions of soluble phosphate, and altogether vastly superior manure, can be got by mixing these large quantities, than by treating little batches.
Mention has already been made of the ill effeot exercised by ferric oxide and alumina, in causing the manure to "go back" in quality. Some experiments made were thought to show that the evil could be remedied by mixing such phosphates in small quantities, so that the heat of the mass should be far less considerable. But it was soon found that, besides producing a manure of very inferior physical qualities, the plan only had the effect of postponing the deterioration in chemical qualities, and did not in any degree prevent it. In working with phosphates whose
" setting " power is good, deptb is not essential in the den ; but in all other cases, it is absolutely necessary, and better results are obtained in proportion as the shape of the den approaches more nearly that of a cube (omitting the corners). One manufacturer, of great experience in using German phosphorites, prefers a den of large area, so that the maunre may run out thin, and cool quickly, and he thinks that this considerably lessens the liability of some manures to " go back." On the other hand, this idea is contradicted by the experiments just alluded to, and, in running the manure out very thin, there is always great risk of destroying the uniformity of the mass, particularly when bones form an ingredient of the manure.
In cases where difficulty is experienced in obtaining a manure that will dry well, a small pro portion of gypsum may be conveniently added, as a drier, if the phosphates used are of such high quality as to bear that admixture, and at the same time to yield at least 25 per cent. of soluble tribasie phosphate of lime.
The production of so-called " turnip manures " and "dissolved bones" is achieved by the addition to the dry materials, before mixing, of such nitrogenous or ammoniacal matters as have already been mentioned. When hones are used, they are generally intro duced in fragments of the size known as " half-inch " ; in this form, they are so slowly acted upon by the acid that they remain in specks throughout the mass, plainly visible to the eye of tho suspecting farmer.
Screening.—After allowing the manure to heat for 24-36 hours, it is dug out, and put through a machine for reducing it to a pul verulent form. The best machine for this purpose is that known as Carr's disintegrator. The material in passing through the machine is subjected to percussion from the bars of the cages which compose the latter, and which re volve in alternate directions at a high rate of speed. A easing or hood is necessary to pre vent the scattering of the material, from the centrifugal force it acquires whilst under operation. The power required to drive the machine varies from 8-H.P. upwards, accord ing to its speed, the quantity delivered, and the nature of the material. It is scarcely necessary to add that only a thoroughly dry manure can be disintegrated in this machine, nor indeed in any other, except with the greatest trouble.