The two great practical difficulties that have been met with in this very simple and ingenious proe,ess are (1) the irregular working of the decomposer—the liability of the copper salt to volatilize or choke up tha interstiees of the marbles, or whatever paeking may be used ; (2) the irregularity of the strength of the bleaching powder, owing probably to the dilution of the chlorine. To meet the un cortain volume of gas entering from the decomposing pans it is sufficient to organize alternate chargings, so that the amount shall be kept as uniform as possible. The activity of the sulphate of copper can be renewed from time to time by the injection of steam, after allowing the decomposer to cool down. A fresh solution of copper salt is made in this way, and distributed over the surface of the packing. .The only bitherto insuperable objection to the process has been the uncertain character of the bleaching powder. Sometimes it will coma well and rapidly. up to strength ; the next compartment .or next charging will obstinately refuse to rise above 30 per cent. In the writer's opinion the solu tion of this difficulty will be found to lie in a better preparation of the lime, and more especially in regulating the amount of water in the slaking operation.
The exaet character of the chemical changes that take place in the decomposer are still very imperfectly understood. Deacon and Hurter have been of the opiuion that the sulphate of copper undergoes no change, but that its action is simply mechanical. They claim to have established,— 1. That with the same mixture of gases, and at the same temperature, the amount of hydro chloric acid decomposed by the aid of a molecule of the copper salt in a given time depends upon the number of timeethe molecules of mixed gases are passed through the sphere of action of the copper salt. Conversely, that the activity of a molecule of copper salt depends upon the speed with which fresh matter is presented to, and the products are removed from, it.
2. That in porous matters, the opportunities of action increase with increased velocities of the current of gas in nearly direct proportion.
3. That, other conditions remaining the same, the percentage of hydrochloric acid decomposed in any given time varies with the square root of the proportionate volume of oxygen to hydrochloric.
acid ; and conversely.
4. That the chloride of copper formed bears no definite proportion to the chlorine produced.
5. That as the sphere of action includes molecules not in contact with the copper salt, hydro chloric acid must he decomposed under circumstances where the union of either element with the copper salt is impossible, i.e. that the decomposition must in part, if not entirely, be caused by the resultant of the forces engaged, and therefore direct from 2HC1 0 to 201 + 1120.
Many other processes for the production of chlorine have been from time to time proposed, besides those to which prominence has been given. Those of Deacon, the acting upon salt with sulphuric anhydride and air— SO, + 0 + 2NaC1 = Na,S0, 4- C1,— Lalande and Prud'homme, Manghau, Tessie du Mothay, Thilbierge, Dimlop — decomposing a mixture of nitrate of sodium and chloride of sodium with sulphuric acid—and Shanks may be men tioned. None of these methods have, however, been worked to any successful issue upon a large scale, but those of Dunlop and Shanks deserve a few words of notice. The former is still in open. tion, the latter has only recently been abandoned. In Dunlop's process the materials were decom posed in a cast-iron cylinder, the following equation representing the reaction :— 2 NaC1 NaNO, + 3 H,S0, = 3 NaHSO, 012 + 3 H20 + NO,.
A mixture of salt with the bisulphate—drawn off in a liquid state—yields, upon furnacing, ordinary salt-cake and hydrochloric acid. The gaseous products are conducted through a series of leaden Woulfrs bottles, partly filled with strong sulphuric acid, which absorbs the nitrous acid while the chlorine is passed on to the bleaching-powder chambers to be absorbed by lime.
Shanks' process consists in decomposing chromate of lime by hydrochloric acid, yielding chlorides of calcium and chromium and chlorine. Thus 2 CaCrO, 16 HC1 = CrCla 4- 2 CaC12 -I- 8,0 + CI,.
The residual liquors are neutralized with lime, as in the Weldon process, and treated with excess of milk of lime. They are then allowed to settle, the chloride of calcium is drained off, and the residual sludge furnaced at a low heat with access of air. Chromate of lime is re-formed and ready to be used over again in the stills.