The decomposition of a solution of common salt, and its conversion into chlorine gas and caustic soda solution by means of an electric current, has long been a study with electro-chemists. Experimentally it has often been effected, but so far as we are aware, the success of this method of production has never until now been demonstrated on a sound commercial basis. The solution of this important industrial problem is due to Mr. James Greenwood, who has been engaged in the development of electro-chemical processes for many years. The outcome of this is that Mr. Greenwood has now perfected an electrolytic process for the direct production of caustic soda and chlorine, as well as other chemical products, the operation of which we recently inspected at Phoenix Wharf, Battersea, London. One of the special features in connection with Mr. Greenwood's new departure is the novel and ingenious method by which the electrolyzed products are separated, and their recombination rendered impossible. This object is attained by the use of a specially constructed diaphragm which is composed of a series of V-shaped glass troughs, fitted in a frame within each other with a small space between them, which is lightly packed with asbestos fiber.

Another important feature of the apparatus is a compound anode which consists of carbon plates, with a metal core to increase the conductivity. The anode is treated in a special manner so as to render it non-porous and impervious to attack by the nascent chlorine evolved on its surface. No anode appears ever to have been invented that is at all suitable for working on a large scale, and the successful introduction of this compound anode, therefore, constitutes a marked advance in the apparatus used in electrolytic methods of production.

The apparatus by which the new process is being successfully demonstrated on a working scale has been put up by the Caustic Soda and Chlorine Syndicate, London, and has been in operation for several months past. The installation consists of five large electrolytic vessels, each of which is fitted up with five anodes and six cathodes arranged alternately. The anodes and cathodes are separated by the special diaphragms, and each vessel is thus divided into ten anode or chlorine sections and ten cathode or caustic soda sections. The anodes and cathodes in each vessel are connected up in parallel similar to an ordinary storage battery, but the five electrolytic vessels are connected up in series. The current is produced by an Elwell-Parker dynamo, and the electromotive force required to overcome the resistance of each vessel is about 4.4 volts, with a current density of 10 amperes per square foot of electrode surface. The anode sections, numbering fifty altogether, are connected by means of tubes, the inlet being at the bottom and the outlet at the top of each section. The whole of the cathode sections are connected in the same manner.

In commencing operations, the electrolytic vessels are charged with a solution of common salt, through which a current of electricity is then passed, thus decomposing or splitting up the salt into its elements, chlorine and sodium. In the separation of the sodium, however, a secondary action takes place, which converts it into caustic soda. An automatic circulation of the solutions is maintained by placing the charging tanks at a slight elevation, and the vessels themselves on platforms arranged in steps. The solutions are pumped back from the lowest vessel to their respective charging tanks, the salt solution to be further decomposed and the caustic soda solution to be further concentrated. The chlorine gas evolved in the fifty anode sections is conveyed by means of main and branch tubes into several absorbers, in which milk of lime, kept in a state of agitation, takes up the chlorine, thus making it into bleaching or chlorate liquor as may be required. If the chlorine is required to be made into bleaching powder, then it is conveyed into leaden chambers and treated with lime in the usual manner. The caustic soda formed in the fifty cathode sections is more or less concentrated according to the particular purpose for which it may be required.

If, however, the caustic soda is required in solid form, and practically free from salt, then the caustic alkaline liquor is transferred from the electrolytic vessels to evaporating pans, where it is concentrated to the required strength by evaporation and at the same time the salt remaining in the solution is eliminated by precipitation.

Such is the method of manufacturing caustic soda and chlorine by this process, which will doubtless have a most important bearing upon many trades and manufactures, more particularly upon the paper, soap, and bleaching industries. But the invention does not stop where we have left it, for it is stated that the process can be applied to the production of sodium amalgam and chlorine for extracting gold and other metals from their ores. It can also be utilized in the production of caustic and chlorate of potash and other chemicals, which can be manufactured in a state of the greatest purity. A very important consideration is that of cost, for upon this depends commercial success. It is therefore satisfactory to learn that the cost of production has been determined by the most careful electrical and analytical tests, which demonstrate an economy of over 50 per cent. as compared with present methods. Highly favorable reports on the process have been made by Dr. G. Gore, F.R.S., the eminent authority on electro-chemical processes, by Mr. W.H. Preece, F.R.S., and by Messrs. Cross & Bevan, consulting chemists. Dr. Gore states that the chemical and electrical principles upon which this process is based are thoroughly sound, and that the process is of a scientifically practical character.

Should, however, the economy of production even fall somewhat below the anticipations of those who have examined into the process very carefully, it can hardly fail to prove as successful commercially as it has scientifically.