case with other sorts. The fact that there is a difference of power at all in what is called the catalytic substance, implies a difference of process.

Thus chromic oxide has the same kind of power as the sulphate of copper in transforming hydrochloric acid and oxygen into chlorine and water, and, indeed, its use was patented by a Belgian as an improvement on the latter substance. But while it has the advantage of nonvolatility, its use is open to the same objection that is true of any form of the process, viz., the production of dilute chlorine.

Before passing on to this subject, however, we may add that among other substances which act like sulphate of copper and chromic oxide, are platinum black or spongy platinum, manganese compounds, etc. Platinum is best used by soaking brick, or asbestos in a solution of its chloride; if then the substance be subjected to a red heat, the platinic compound is decomposed, leaving a film of the metal over the surface. Any porous substance has the catalytic power, only in a less degree than when supplemented by some such agent as sulphate of

copper.

To return to the study of the chlorine produced by this process. It is associated with all the nitrogen of the air, and any excess of air that may have been used, and for a long time it was thought that such dilute chlorine could not be used for making strong bleaching powder, that is, it would present difficulty in its absorption by lime. Nevertheless, by careful regulation of the amount of air used, a gas can be obtained which will produce 'bleach' of full strength, under conditions to be hereafter described.

A perfect decomposition of the hydrochloric acid is never effected, and hence it becomes necessary to wash out the hydrochloric acid attached to the chlorine en route for the chambers. Hence, in any case, Weldon's or some other process would have to be employed to work up this condensed hydrochloric acid. Of course it could be used by boiling it to generate fresh gas for the decomposer'; but this would entail a large consumption of fuel. After freeing the chlorine from hydrochloric acid, it has to be dried by passing through a tower, wherein it is exposed to sulphuric acid of specific gravity 1.6, and in some works it then passes on to another tower packed with fused chloride of calcium. Here the drying is completed, and the gas is then ready for chamber consumption. But for this purpose the ordinary form of chamber, the consideration of which matter we reserve for our next chapter, cannot be employed. In this place we shall briefly describe the form of chamber used in the Deacon process only.

The stronger and purer the chlorine, the more readily is it absorbed by lime, and vice versa. When dealing, therefore, with such dilute gas as is obtained in the process under description, there must be presented to it a larger surface of absorbing material, and this is done by spreading upon a series of shelves made of slate, lime to the depth only of about of an inch. The lime is charged into the chambers from the top through a hopper, and is then spread upon the shelves by means of a rake having a gauge of § of an inch. These shelves are 6 inches one below the other, and are so constructed that the fresh chlorine first passes over that powder, which is already most nearly saturated, and thence to the less chlorinated

lime. In this way good bleach containing 37 or 38 per cent. chlorine may be readily prepared.

Altogether, the Deacon process, so far, cannot be said to have worked so well as was expected, and no plant yet constructed, has been able to produce that amount of bleaching powder which it was calculated and destined to give.

The various difficulties encountered in the process of working may be thus enumerated:

1. The roaster gas is liable to contain carbonic acid gas, which presents an obstacle in the production of good bleach, forming as it does carbonate.

2. The plant being largely constructed of iron and exposed to high and inevitably varying temperatures, the joints are liable to give way, and admit the products of combustion from the furnace outside.

3. The copper is apt to volatilise as chloride, which condenses in the cooler part of the decomposer, impeding the draught, and involving stoppage for cleaning; while the marble, under such circumstances, requires periodically redressing.

We have endeavoured to sketch this process fairly and truthfully, and if we appear to have laid special stress upon the difficulties inherent in it, we feel justified by the importance of the subject. Nevertheless, the circumstances and conditions under which this process can attain commercial success, may yet be discovered, and thus reward the indefatigable exertions already made to perfect it, by Mr. Deacon and his chemist, Dr. Hurter.

Illustrative of these difficulties, it may be stated that a new anemometer, for measuring the speeds of the gases to the decomposer, had to be invented, and also new

forms of pyrometers for indicating their temperatures. Further, when it is considered that a manufacturer, making 40 tons of sulphate of sodium daily, in order to use all his hydrochloric acid as generated, would have to deal with about 1,100,000 cubic feet of gases, the means whereby they have been enabled to partly accomplish this, reflect exceedingly well on those who have devised them.

In passing from this study, there yet remains one other process to be described before we proceed to some further considerations regarding the production of bleaching powder. The process we refer to, is the subject of a patent by Messrs. James Hargreaves and Thomas Robinson, and has been proposed in reference to the manufacture of hydrochloric acid and chlorine.

One part of potassic or sodic chloride is mixed with two to three parts of oxide of chromium, and with or without magnesium or manganese as oxide, or in the form of a salt, to assist in the production of chromic acid. The mixture is moulded into bricks with water, and then dried. These are then exposed to a current of air, at a temperature which may be expressed as 'red heat,' but below fusion, in a series of furnaces or other chambers. In this way chlorine is generated, and the sodium or potassium converted into chromate. If hydrochloric acid be the desired product, along with the current of air, is used one of steam also. In both instances, if desirable, the air may first be made red hot by use of the air-heating apparatus of the Messrs. Siemens. The equations illustrating the decompositions may be thus expressed:

(1) 2NaCl + Cr2O3+05=2NaCrO4 + Cl2

(2) 2NaCl + Cr2O3 + H20+0=2NaCrO, + 2HCl.

CHAPTER XVII.

MANUFACTURE OF BLEACHING POWDER, BLEACHING LIQUOR, CHLORATE OF POTASSIUM, AND EPSOM SALTS.

BLEACHING POWDER is produced commercially by the action of chlorine prepared by any of the afore-described processes upon slaked lime.

The chlorine gas is first dried by passage through small leaden or other purifiers, packed with coke or flints, kept well supplied with strong sulphuric acid. In many works the chlorine is washed by passing it through water before drying. This is done in order to prevent the occurrence of traces of manganese in the bleaching powder for it is asserted that peroxide of manganese, even when present in very minute amounts, greatly facilitates the spontaneous decomposition of the product, with the evolution of oxygen. This is a matter, however, yet requiring investigation. The choice of the lime used is of much importance, many kinds containing traces of ferric or manganic oxide; and such substances give to the bleach' the induced liability to decomposition already referred to. Such lime is to be identified by its yellow colour. Irish and French limestones furnish, by a process of calcination, most of the lime used in this country, as they give a 'heavy' lime, which is the best sort to employ. Large quantities of limestone are also obtained from Buxton.