2. Smelt with ore - hearths. Formerly water condensation was used, but damage having resulted to the foundations of the works, it was abandoned, and dry condensation substituted. The flues are arranged in a block, and in the form of a zig -zag; 5 tiers of such zig-zags, one above another, having been built up, each tier consisting of 5 passages. The fume enters the nearest passage of the top tier, passes backwards or forwards along the 5 passages, and then drops down to the first passage of the tier below, and so on to the bottom. It then goes through a series of 4 chambers, from the last of which a straight flue conducts it up the side of a hill to the summit, where it terminates in a chimney 120 ft. high; the height of the hill is 110 ft. Following are particulars of the size of the flues and chambers, and the rate of passage of the fume through them: -

The volume of gases passing into the condensing - flues is 926 cub. ft. per minute; hence the length of time required for the gases to pass from the smelting - furnace to the exit at the main chimney is 3 hours 39 minutes. The condensation here is very effectual.

Cookson finds it more economical in the long run to roast the ore before smelting in the ore-hearth, notwithstanding that this process increases the cost of the smelting 34 per cent. Especially in the case of roasted ore, his exit - gases are remarkably free from (solid) fume, whereas in the case of raw ore the escaping fume is greater than that of the roasted ore, even taking into consideration the original quantities of fume made. Following are particulars of the results obtained on smelting 400 tons of ore, all the same assay, viz., 81 per cent., half of which was smelted raw, and half after having been roasted. These particulars are quite reliable, as extreme care was used: -

In the case of raw ore, the "hearth ends" were a large, item, but very small and very much less in the case of the roasted ore. The proportion of fume (including "hearth ends") made is as 1, in the case of the roasted ore, to 3.28 in the case of the raw ore; but the . testing of the exit-gases at the chimney showed that the fame escaping was as 1 in the case of the roasted ore to 3.35 in the case of the raw ore. ■ . 9. Smelt with the ore - hearth. The fume is passed through 3 large chambers, the first with 3 divisions and the others with 4, and the fume is made to pass through these chambers in succession on its way to a wooden chimney attached to the works, and 85 ft. high. The following is approximately the size of the chambers in feet. viz.: -

4. Smelt with the ore - hearth. There is in use here a "condenser" constructed of wood. There are 2 chambers side by side, with'an interval of 3 or 4 ft: between, in which interval is a platform for the use of the workmen. The chambers are each about 84 ft. long by 4 ft. wide and about 12 ft. high, standing in what is virtually a trough of water, and each divided into. 16 divisions by vertical partitions springing alternately from the roof and from beneath the surface of the water near the bottom of the chamber, the partitions springing from the top not reaching the surface of the water, those springing from below not reaching the roof, so that fume in passing through the chambers has to pass up one division and down the next in its course through the 16 divisions. At the top of each division is a colander, into which water is conducted by means of a wooden channel above, and from which the water falls as a shower upon some brushwood, with which all but the lower part of the several divisions is filled. Any solid matter washed out of the fume collects in the water - trough below. There is a provision for the water running off from the trough when it reaches a definite level.

A fan is provided in the flue which conducts the fume to each chamber, and not only drives the fume through the chamber, but acts also beneficially in drawing off fume from the hoods beneath which the ore - hearths are erected in the smelting - house and from the calciners. The 2 fans were in use 24 years before requiring serious repair. From the condenser, a fine, with sloping sides, 5 ft. 8 in. high, 4 ft. wide at the spring of the arch, and 2 ft. wide at the bottom, conducts the fume to a ft wooden chimney 1 1/4 miles off. Wood, however, is an objectionable material for the condenser to be made of, since it allows of leakage.

5. Smelt with the ore - hearth. Water condensation is used here, and is very effectual. An arrangement similar in many respects to (4) is adopted, but the condenser is erected in brickwork, which is better than wood. There is a pair of brick chambers about 20 ft. high, each divided by partitions into 6 divisions, over and under which alternately the 'fume has to pass. The water is supplied in gushes from, a colander above each chamber through the agency of a tip - jack, and instead of bushes, rows of short drain-pipes are arranged on shelves within the condenser to break up the water as it falls through. From the washer the fume passes to 3 chambers of wood - provided within with a zig-zag arrangement and brushwood, and from these, by means of a large flue, to a wooden chimney on the top of a hill; the length of the f1ue is 800 yd. The quantity of water used in this condenser is about 2 tons per minute. About half of the fume is taken out by the condenser, and the other half deposits in the long fine and zig-zags, through which the fumo subsequently has to pass to the chimney. The sulphurous acid appears also to be in great measure washed out.

The arrangement for the condensation is very effectual.

At many works water condensation is little more than nominal, consisting of narrow showers of water passing here and there into the flues. It is sufficient to damp the fume with steam, but not to effect much more. French says that practical results accord well with the theory that rain - drops,, with their comparatively enormous surface tension, are ill adapted to collect the extremely mobile particles of fume they encounter in their descent. It is also less easy to condense the fume proceeding from a reverberatory smelting furnace than from ore - hearths, since in the former case the efficiency of the smelting is dependent upon the strength of the chimney draught, while in the latter the draught need only be moderate, and the supply of air is given by a blower in just the moderate quantity requisite, and is strictly under control. But while the solid element of the fume can be arrested in the way described, long flues have no influence in arresting the escape of sulphurous acid from the chimney, and ordinary modes of washing with water fail to remove more than a moderate proportion of it. Probably such washing will remove more when the ore - hearth is used than when the reverberatory furnace is used, since in. the former case the acid is less dilated with air.