Abstract of a paper read at a meeting of the Cold Storage and Ice Association, London, Eng., by Mr. Charles Page.

You are, no doubt, all familiar with the form of a gas retort in which coal is distilled for the production of what, in spite of electricity, may still be described as our chief artificial illuminant. From this retort the crude gas and tar ascend by a pipe, which passes into the hydraulic main, a sealed trough containing water, where the tar is separated from the gas. The latter, still in the crude state, is led away by pipes, to go through the various processes of purification necessary to make it a good and innocuous illu-minant. Of the impurities, ammonia forms an important part, and in order to remove it the gas is passed through a scrubber, of which the most common form is a series of towers containing coke, through which a constant flow of water is maintained. As water readily absorbs ammonia, this impurity of gas is given up during the passage of the crude gas through the scrubber.

The water of the last of the series of towers, where very little ammonia is present, is Used over again in the rest of the series until it contains about two per cent of ammonia. This ammoniated water, termed commercially gas liquor, invariably contains other impurities of coal gas, including sulphur and a small percentage of tar, which will have passed over with the gas from the hydraulic main. This gas liquor, together with the tar and the water from the hydraulic main (which will also contain ammonia), is collected in suitable receivers, most commonly underground tanks, where the tar sinks to the bottom, and the two can be separated subsequently by suitably arranged pumps.

At this point it would be interesting to note some of the commercial aspects of the production of ammonia. You will have remarked that ammonia is a by-product of the manufacture of gas, and when I tell you that a ton of coal will yield only about four to five pounds of pure ammonia you will see at once that it could not be produced and sold at the current market price as a main product. Coals of different origin vary greatly in yield of gas and its by-products, but taking the average yield of the coal used for gas-making in this country, it may be stated roughly that one ton of coal distilled by the most modern process, will yield about 11,000 cubic feet of gas, 200 to 250 pounds of tar, four to five pounds of pure ammonia, twenty to twenty-two pounds of sulphur, and thirteen to fifteen hundred weight of coke.

The by-products, or residuals, as they are termed by gas engineers, form a very important part of the industry of gas-making, and naturally great attention is paid to them, both in the selection of coal and in the methods of distillation. So important a part do these residuals play, that in some places, where coal is cheap and the production of gas large - the town of Sheffield, for instance - the total cost of the coal is covered by the yield of the residuals. Those of you who are share-holders in gas undertakings will realize, therefore, what an effect the price of residuals, as the by-products are termed, will have upon your dividends.

And for all of us the point is an interesting one, as it exemplifies how one industry is dependent upon another in a way which by no means appears upon the surface. You would not think that the price you pay for the ammonia which you use for refrigeration has any sort of connection with the dress which your wife wears, which is dyed by a tar product possibly made from the same ton of coal as some of your ammonia, but it is an economic fact none the less.

I would mention here that ammonia is also produced in the processes of making coke for iron smelting and of distillation of shale for the manufacture of oil as carried on in Scotland.

These are important sources of ammonia, and the methods of obtaining it are much the same as those I have described. Water is the vehicle, and what may be termed the raw material, which is used by the manufacturers of ammonia, is gas liquor, containing only a small percentage of ammonia.

To return to the process of manufacture. I have shown you how gas liquor containing about 2 per cent of ammonia is obtained. This ammonia is partly free and partly fixed - that is, part of it will evaporate, and it is this portion which gives the strong odor to the liquor, while part of it is held in solution by the sulphur which also comes over with the other impurities of the crude gas. The relative portions of free and fixed ammonia vary according to the nature of the coal used and the condition of distillation. This gas liquor contains numerous impurities, which would go over with the ammonia if the latter were simply distilled, and it is the business of the manufacturers of anhydrous ammonia to get rid of these impurities, and so to produce an ammonia which will contain nothing which will have any injurious effect upon the most sensitive parts of a refrigerator plant, and which will be easy of compression and rapid of expansion. The surest method of accomplishing this is to make first a solution of this salt and drive the ammonia off again before drying and compressing it.

This brings us to the manufacture of sulphate of ammonia, a perfectly odorless salt, containing about 25 per cent of ammonia. In this process the gas liquor is passed into the top of a column, like a boiler placed on end, divided at intervals by plates which are perforated in the center and at alternate sides, and the perforations are so guarded that the gas liquor rests on each shelf to a certain regular depth before it can pass to the plate below. While the liquor thus descends from plate to plate through the tower, steam is admitted at the bottom and ascends through the center holes of the plate though the tower, (or, in some processes, in a separate tower to which the liquor is carried) milk of lime is introduced, and this sets free the fixed ammonia, which in like manner is carried up with the steam. The number of plates in the tower and the treatment by lime is governed by experience of the liquor being worked, and the outflow of water is tested from time to time to see that there be no waste of ammonia. The residue should not contain more than .002 per cent, or about one-thousandth part of ammonia which the gas liquor contained on entering the tower. The ammonia carried up by the steam is conducted by a pipe from the top of the tower to the saturator, a vessel containing sulphuric acid of the proper strength, which fixes the ammonia and forms sulphate of ammonia, precipitating it in the form of crystals, which are fished from the saturator and allowed to drain.

Having now imprisoned our very volatile gas in the form of sulphate of ammonia, it can be left exposed to the air for any period without detriment so long as the salt be kept fairly dry.

This, then, is the most convenient form in' which ammonia can be transported, and it is in this form that several hundred thousand tons per annum are used for fertilizing purposes. But for our purpose we need the pure ammonia gas, and this is obtained by dissolving the sulphate of ammonia and adding milk of lime to it, which again sets the ammonia free. The solution is treated in much the same way as the gas liquor, but the resulting gas is naturally much more free from impurities than in the first process. In this process there is considerable loss of ammonia, for it is found that even when an excess of lime is used - by an excess I mean more than the quantity required chemically to combtne with the sulphuric acid and form sulphate of lime, which is the process by which the ammonia is set free - with an excess of acid and the employment of mechanical agitators it is impossible to recover all the ammonia. A yield of 90 per cent is in practical working considered a good yield.

This ammonia is now considered a very volatile gas, and throughout the remainder of the process great care has to be exercised to guard against loss by leaky joints, or accidents, or carelessness of workmen. It is first conducted to a condenser, where the gas is cooled, and then to purifiers and driers. In this stage of the process lime plays an important part, as it absorbs the moisture, while it has no affinity for ammonia. To ensure absolute dryness the ammonia gas is finally passed through a cooling tower which is itself refrigerated by means of anhydrous ammonia from the compressor, so that any remaining moisture is frozen out of the gas.

After the process of purifying and drying, the ammonia passes to the compressor, which works at a pressure of about 150 pounds, and from there it goes to a condenser, where it is contained until filled into cylinders for conveyance to the buyers.

A word about these cylinders, which form a very important and probably the most expensive part of the manufacturer's stock in trade, will not be without interest. They are made from tubes of toughened steel, and when new, after the valves are fitted, are subjected to a test by a hydraulic pressure of 1500 pounds to the square inch. They are thus absolutely safe even in the hottest climate when properly filled with anhydrous ammonia, but to insure this it is necessary that the quantity should not exceed five-eighths of the water capacity of the cylinder.-"National Engineer."