When the manure is to be dug out of the den, the latter is ventilated by removing the roof, if temporary, or by opening windows provided for that purpose, if it bo a permanent covering. As the vapoUrs generated during the mixing of the manure, and immediately after its outlet from the mixer, are those chiefly to be dealt with, attention is mostly confined to means of drawing them away and rendering them innocuous.
No very complete analysis has yet been made of the constituents of the gases evolved in the manufacture of artificial manures. In the case of ordinary superphosphate, fluoride of silicon is formed by the action of the sulphuric acid upon the silica and fluoride of calcium contained in the said phosphates. The fluoride of silicon, in the presence of condensing watery vapour, is resolved at once into hydrated silica and hydrofiuosilioic acid, thus-3SiF, = + 2 (2HF, Dr. Adams has also oonelusively proved the vapours to contain arsenic, from the arsenical sulphuric acid used, most of the acid employed for manure-making being derived from pyrites. Without doubt, some of the arsenic is evolved as arseniuretted hydrogen, from the action of the acid upon the iron portions of the interior of the mixer ; but the greater part is probably in the form of chloride of arsenic. The proportion of the latter will be commensurate with the amount of chlorides decomposed in the mixing, and, estimated as arsenious acid, varies from 2 to 10 oz., and even more, for each ton of manure made. When organic matters are added to the raw constituents of the manure, additional offensive vapours are generated, of very various characters. When much salt is present, the production of hydrochloric acid vapour will be great. The odours from manure-works are carried considerable distances, extending sometimes to over 4 miles.
Of the offensive vapours given off during mixing, some are condensed by cold, some dissolve in (or are decomposed by) water, and the remainder are destructible by fire. The application of these s gen ts — cold, water, fie 1 OC.
itself into long flues, water-towers or " scrubbers," and fur naces, usually assisted by motive power, snob as that produced by the draught of a tall ohimney, or by a fan.
In simple super phosphate-making, a long flue seems to answer every purpose, and has been success fully adopted in some of the largest works. Its object is, by cool ing, to promote con densation of the steam, and conse quent deposition of the nyaronuositicie acid, and other matters, before arrival at the chimney, by which they would otherwise escape into the atmosphere. In these flue deposits, is found a notable quantity of arsenic. An illustration of a thoroughly efficient arrangement is given in elevation in Fig. 952. The mixer A is connected
by a short flue a, 12 in. square, with a wooden chamber B, about 18 ft. long, and 3i-4 ft. wide and deep, divided at equal intervals by partitions b, springing alternately from top and bottom. At the bottom of each partition thus formed, is a door c, by which the deposit is periodically re moved. The chamber B opens into the top of Lb square brick tower C, about 14 ft. 6 in. high, and 2 ft. 7 in. in diameter, receiving at d the vapours arising from the pit or den D. In the tower C, more silica is deposited. Adjoining C at the bottom, is a shorter tower E, with a communication between the two at e, about 4 ft. above the bottom of the former. The deposit accumulated in C is thus prevented from choking the passage, and is removed by a door at f. From E, the vapours traverse an underground flue F, 150 ft. long, terminating in ZS chimney. The chief deposition takes place in B and C, the former being cleaned out twice a week, and the latter once a month. Beyond the first 15 yd. of the flue, the deposit is scarcely appreciable, and the flue is only cleaned out once a year. At the works where this plan is in operation, the manure made averages 100 tons a week, about half the raw phosphate being bones. At another works, making 300 tons a week, and employing nearly all mineral phosphates, the total flue is 440 ft. long.
A second method, adopted successfully in some works, is to condense the vapours by the direct application of cold water. This is effected either by a shower or cascade, or by means of a " scrubber," i.e. Lb tower partly filled with material over which liquid is made to fall. A most efficient example of the shower or cascade arrangement is shown in Fig. 953. From one end of the mixer, an opening, the entire width of the mixer, and about 15 in. deep, communicates with a wooden channel a, into which emerges a similar connection b from the den c. At about 3 yd. from the mixer, the channel a communicates with the upper part of a water-tower d, where the vapours meet with a shower. The tower is of wood, 3 ft. square, and 18 ft. high. At the top, is a tank e, with a perforated zinc bottom, fed by a 3-in. tap f. Within the tower, a series of wooden shelves spring alternately from opposite sides, with flanges to determine the flow of water towards their centres. At the bottom of the tower, is a cistern h, whence the water flows over a ridge into the drain i. The unabsorbed vapour is drawn off by a fan, and conducted into the boiler-fires of the works. With an abundant water-supply, nothing is more effective than this plan.