produces the heaviest crop; but on the Figgate Whins, mere irrigated sand, the first crop is earlier in the season—a matter of such consequence, that although the annual yield is less, the rent paid for these plots is about as high as for the plots producing the heavier crop. The rental of the Figgate Whins, previous to the irrigation, was, I have been informed, about twenty shillings per acre; while, when irrigated, parts have been let for some years at L.40 per acre. The only works having been the levelling of the sandy hillocks, and formation of channels for the sewage-neither of them very costly operations-and the annual outlay being small, the increased annual value of that land may be stated at not much less than the difference between the two sums.

It might be an interesting speculation, to consider how far the cost of the works necessary for collecting and removing the sewage from the district of the city, draining towards Craigentinny, might have been defrayed by the advance of rent obtained by the disposal of the sewage in irrigating the land along the course of the stream. The cost of the whole sewerage works (including many of the branch drains), constructed within the district in the city which is drained to the Craigentinny Burn, may be stated at L.96,000. Assuming that the annual rent of the 250 acres irrigated was L.5 per acre on an average, previous to being laid out for irrigation, while the rent was raised to L.25, then the difference, L.20 per acre, is the annual value of the irrigation. There being 250 acres, gives L.5000 as the return, or upwards of 5 per cent. on the cost of the sewers.

The produce of the various irrigated meadows round Edinburgh is sufficient to supply the present demand for grass; necessitating any further application of the sewage to some other kind of crop, unless a more extensive market is obtained for the grass produced.

It may prevent misapprehension to state, that in no case is the whole sewage of any of the streams absorbed, even when the water is applied as largely as possible to all the meadows along their course; and no irrigation is carried on from September till February (except at the Figgate Whins), or while the sewage is much diluted with rain-water. At

such times, the whole sewage passes to the Firth of Forth without being used.

The city derives no pecuniary advantage from the irrigations; and, indeed, the authorities proposed to try, in the year 1839, whether the system could not be put an end to, the exhalations being considered by some to be injurious to health. However this may be, there is no doubt that the feeders or open streams for the sewage are most offensive; but so far as can be judged by the senses, it is from them alone that the emanations come. Great improvements could therefore be easily made, by forming channels of some comparatively non-porous material.

Having now described the system adopted for the collection, removal, and disposal of the dry or solid, and also of the fluid refuse of the city, I may add, that the description has been necessarily condensed, so as to bring this paper to a suitable length. I have endeavoured to describe the system exactly as it at present exists, and I trust that the following conclusions with reference to it have been satisfactorily established, namely:

First, That the prompt daily removal of dry or solid refuse from dwellings, which is the system adopted in Edinburgh, is incomparably superior to the system of storing such refuse for some time previous to its being removed.

Second, That the only part of the dry refuse not disposed of usefully, is the mud from the macadamised roads.

Third, That besides removing nuisance from the river, mill-leads, and harbour, the works for the purification of the Water of Leith are necessary in the view of the sewage being utilised.

Fourth, That sewage was first utilised by its application to the grass lands adjoining Craigentinny Burn; and,

Fifth, That the irrigation near Edinburgh cannot be profitably extended, without a more extended market for the grass produced, and that, therefore, some simple mode. has yet to be discovered for profitably applying to other crops the valuable fertilising properties contained in the

sewage.

Description of a Revolving Apparatus for Washing out Ammonia and other Impurities from Coal Gas, in the process of its Manufacture, and applicable to the Purification of Air, and for other Sanitary purposes. By JOHN REID, F.R.S.S.A., Engineer and Manager, Edinburgh and Leith Gas Company.*

The contrivance which I have now the honour of bringing under the notice of the Society of Arts was put in operation in September 1865, at the works of the Edinburgh and Leith Gas Company, in Leith, and has been continued in constant and successful operation day and night ever since. It has in every way realised expectation, and in many respects surpassed it. The machine is employed, and takes its place in the gas-works as a washer, being now almost universally employed by gas companies for assisting in the purification of gas for illuminating purposes.

Before describing the arrangements of my apparatus, and mode of working, it may be useful to explain its special function, as well as the usual structure of washers in general use in large gas establishments. In doing so, however, I need not enter, even if I were competent, into the chemistry of the numerous complicated products from the distillation of gas coal. It is sufficient that I merely indicate generally the nature of the substances in combination with the crude gas which it is the business of the gas engineer to have removed, and as far as possible utilised, before the finished gas is passed into the gasholder for distribution.

The first process in the manufacture is the distillation of the gas coal in brightly-heated close retorts-usually made of fire-clay. In the course of three to four hours, a charge of about 220 lbs. of Scottish cannel coals in each retort will have given off its volatile products, after which the residual coke is drawn out and replaced with a fresh charge of coal. The component elements of the mineral are thus broken up into a variety of new forms, gaseous, liquid, and solid.

* Read before the Society, and illustrative diagrams exhibited, 25th February 1867. Awarded the Society's Silver Medal and Plate, value Five Sovereigns.

Some of the newly liberated elements enter into new combinations, forming substances unknown as such in the original coal. These consist chiefly of compounds of hydrocarbons, sulphuretted hydrogen, ammonia, carbonic acid, carbonic oxide, and compounds of sulphur and carbon; and when the distillation is completed, it is found that solid carbon, with more or less earthy and other matters,—depending on the kind of coal,—is all that remains in the retort. Out of 100 lbs. of coal thus treated, generally about onehalf of its weight will be found to have gone off in the form of gas and vapours. These on passing onwards, and being cooled down in condensing apparatus, deposit something like one-half of their weight in the form of coal tar, tar oils, water, solid and liquid ammonia, &c.

Leaving these grosser substances behind, the gas passing onwards is still tainted with impurities requiring removal. These consist mainly of sulphuretted hydrogen, carbonic acid, and ammonia, all in a permanently gaseous condition, and must as far as possible be removed before the gas is fit to be used for illumination by the consumer.

Hydrated lime and lime-water, or both, are the agents chiefly employed in Scottish gas-works for taking up the sulphuretted hydrogen and carbonic acid, and they do so effectively when the apparatus is sufficiently adapted to the work. But it often happens, that while every trace of the former impurity has disappeared, the gas will be found not entirely free of the latter, especially in the smaller works, where washers are not employed.

The third impurity, gaseous ammonia, is usually removed by liberally washing the passing current of gas with water, with which it readily combines.

Dilute sulphuric acid has been employed by Mr Croll of London for combining with the ammonia at this stage, and forming a solution of sulphate of ammonia. But it has not met with general adoption, for various reasons; but mainly, I believe, on account of the corrosive action of the acid, which has a destructive effect upon the metals forming the apparatus. Water alone is found suitable, and is now most usually employed.

This process usually takes place immediately before the gas is brought into contact with the lime, and has been generally employed in gas-works for the last quarter of a century. But it has been observed within a more recent period, that other important advantages are gained by the use of this process. When worked up to sufficient ammoniacal strength, it becomes an efficient agent in the removal of carbonic acid from the gas, and effects a saving in lime. Another object of not less value is subserved at this stage of the manufacture. It has been mentioned, that in the process of condensation certain substances are left behind in the condensers. Among these the tar and oils, which vary greatly in their character, according to the degree of temperature at which they pass from vapour to liquid. The denser varieties fall first, followed by lighter oils of greater volatility. Those formed towards the close of the condensing process seem as if in a manner undecided whether to continue as liquid or to reassume the gaseous form. These light oils contain the very richest illuminating elements of gas; and it has been found that if they are again brought into contact with the gas in the washing apparatus, the gas has the property of taking up a good deal of these hydro-carbon vapours, which re-form themselves into permanent gas, and thus to a considerable extent restore the illuminating elements previously parted with and left behind in the condensers.

Various forms of apparatus have been devised for effecting this washing operation, which must be effected upon the stream of gas as it rapidly passes onwards from the retorts, and with as little impediment to its free passage as possible. One form was for long, and is still, continued in many gas-works, consisting of an oblong iron chest, say 25 feet long, by 6 feet wide. The top of the chest being off the level, one end being seven or eight inches lower than the other, an elevated chamber is constructed at each end as entrances and exits for the gas. The vessel is charged with such a body of water, that the depressed end of the cover is submerged some seven or eight inches. The gas being introduced within the chamber at the lower end,