The " scrubber" system is, however, in more general use. Its arrangement is shown in eleva tion, plan, and vertical section, in Figs. 954, 955, and 956. Leading from the mixers and the dens, are square wooden flues, terminating in a fan, which draws the vapours from the mixers and dens, and forces them into the condenser. This latter consists of a brick chamber A, standing in a strong leaden tray a. A range of lead-covered iron bars B is built into the brickwork, and supports the two first racks b, which are notched to receive the loose square wooden bars c. The racks are so disposed that the alternate sets of bars c are at right angles to each other. Upon each row of bars, are laid packing-pieces, to carry the next set of racks, and this is repeated to the top. The whole of these racks may be removed for cleaning, and replaced through the door d. At the top of the tower, is a perforated leaden tray e, supplied with water by a tap f. The water falls in a shower upon the intercepting bars within the condenser, and passes between them to the bottom of the condenser, whence it overflows at g into a drain. The vapours from the mixers and dens enter the condenser near the bottom, and beneath the bars B, by means of the wooden flue and pipe h i; and, after passing the condenser, escape near the top, by a short flue k, leading to the chimney of the works. The efficiency of this arrangement seems to depend less upon the flow of water, than upon the obstruction offered to the passage of the vapours, affording time for the decomposition of the fluoride, and the arrest of the products of the decomposition ; but the water undoubtedly renders valuable assistance. The escaping vapours show no trace of fluorine compounda; but still contain an appreciable quantity of arsenic.
A still more effective form of scrubber is shown in Fig. 957. The condenser consists of three vertioel chambers of brickwork A B C; the first contains nothing, but the second and third are packed in their lower portion with perforated bricks a, laid evenly, but not too closely. Above the three chambers, is a capacious cistern D, for supplying water to the condenser. From this cistern', the water flows by pipes b into each of the three chambers. There is an excellent arrangement for distributing the water, which is shown in detail in Fig. 958. Tho bottom of the pipe b is open, but capable of being closed partially or completely by means of a plate or button c, which can be raised and lowered by a screw-rod d, worked from above, and guided into a central position at e. When the plate c is slightly lowered, the water, entering the pipe b from the tank D by the perforations at f, flows out in a thin sheet resembling an open umbrella, and falls down through the chambers, being determined by the shelves g towards the centre of the chambers ; then, passing through and between the perforated bricks a, escapes by the drain-pipes h. The vapours from the mixers and dens enter the upper part of the first chamber A by means of a 12-in, pipe i, then pass below, as shown by the arrow, into the second chamber B, and, after passing through the bricks, enter the third chamber C, above the sheet of water, with and among the spray from which they descend through the bricks, and escape by the flue, some 50 ft. long, which conducts them to the furnaces shown in Fig. 959.
These furnaces are in duplicate, and provided with dampers at a, so that the vapours may be directed into either at will, on their entering from the condenser by the flue b. After passing through the fires c, the vapours descend by the flue d, and pass away to the chimney of the works. As a means of testing to what extent the vapours are deodorized, a small chimney e is provided ; by closing the damper placed at the floor-line in the flue d, and removing the luted cap f, the vapours may be smelt. This combined arrangement of condenser and fire is perfectly efficient, and when kept in working order, the escaping vapours give no trace of arsenic, or other deleterious substance.
The result of these observations is to show that, in the case of manure made from ordinary mineral phosphates, the prevention of nuisance is efficiently ensured by affording the time, space, and other conditions necessary for the decomposition of the vapours, and the deposition of the condensed products ; but that where animal and vegetable substances are used in the manure, not only must these precautions be rigidly observed, but complete success can only be obtained by passing the vapours through fire, to destroy their organic constituents. By the adoption of these combined
measures, the operation of making the manure may be rendered absolutely innocuous and inodorous.
There remains to deal with the odours which arise during the subsequent operations of digging out and screening the manure. It has been suggested that, by leaving the manure for a much longer time in the den, so that the im prisoned gases might have an oppor tunity to condense, all smell would be avoided ; but this is a condition which would be practically impossible in many works, and onerous in all. The only practical solution of the difficulty seems to be by enclosing the den, and the apartment where the screening takes place, and to draw off all the vapours set free in these apartments, by means of a fan, passing them through a condenser and a furnace, as already described.
General Considerations.—The import ance of artificial manures in modern agriculture cannot be overrated, by far the greatest proportion of the vegetable products of all civilized countries at least being grown by their aid. It would be difficult to assign any distinct locality where this branch of chemical manufacture is carried on. The universal distribution of the agricultural industry furnishes sufficient reason for this. Perhaps it is most largely conducted in the immediate neigh bourhoods where the raw materials are mined or quarried, and it is safe to assume that every sulphuric acid manufacturer is more or less engaged in making artificial manures ; at the same time, there are a great number of smaller capitalists, who buy the raw materials, both mineral and acid, and supply local needs. The manufacture is a creation of the last 25 years, and is always spreading ; it will no doubt continue to grow until some better means is found for economizing that great natural fertiliser—Sewage. The sum of money invested in the manufacture must amount to several millions. When the manure-manufacturer does not make his own sulphuric acid, the capital required is very small, as compared with other manufactures. Moreover it may be conducted on any scale, large or small. Legislation concerning the conduct of the manufacture is in a transition state ; but probably next session will find manure-factories under the Noxious Vapours Act, which is not likely, however, to impose any restriction upon the trade, save the prevention of nuisance, which may be easily accomplished. The commercial prices of the manufactured articles vary so much that it is not easy to fix upon a general figure. They are usually sold on a basis of containing a certain proportion of the actively fertilizing principles—the soluble phosphate of lime, the insoluble phosphate of lime, the potash, and the nitrogen (as ammonia), being the elements which enter into the estimation. The values set upon these ingredients are approximately as follows: Soluble phosphate iu bone manures, 4s. 6d. per unit per cent. ; soluble phosphate in mineral super phosphates, 4s.; precipitated phosphate, 3s. 6d. ; insoluble phosphate, as bone, or from guano, 2s. 6d.; mineral phosphate, up to 7 per cent., Is.; potash sulphate, 3s. 6d.; ammonia, 20s.; insoluble phosphate in good "dissolved bones" (when precipitated phosphate is not reckoned), 2s. 9d. This scale is adopted by Alfred Sibson, F.C.S., of 23, St. Mary-Axe, who is well known as an analyst of manures and feeding-stuffs. As between manufacturer and consumer, it is probably the most equitable of any. It assumes that the manures are sold under the conditions usually prevailing in agricultural districts, the article being in dry, powdery condition, supplied in bags, and carriage paid, and credit being given. The two prices for bone phosphate and mineral phos phate are not generally recognized by analysts ; but the justice of the plan is evident, from the greater cost to the manufacturer of phosphate in the form of bones, and the greater expense in their manipulation. Many chemists, also, do not estimate the " precipitated " phosphate, or soluble phosphate which, by long keeping, has reverted to an insoluble condition, an occurrence frequently experienced, especially with bone-manures ; yet the unfairness of not admitting the distinction is manifest. The greatest proportion of the artificial manures sold in this country pass first through the hands of commission agents, before reaching the actual consumers, the farmers.