ELECTRO-CHEMISTRY 498 ELECTRO-CHEMISTRY together as conditions permit, and are made large in area. The most common application of elec- tricity to industrial chemistry is prob- ably in the manufacture of bleach and soda from common-salt. As has been stated, when brine is electrolyzed, sodi- um and chlorine are produced. Sodium, however, reacts violently with water, and cannot exist, as such, in aqueous solution. As soon as formed, it com- bines with water to form sodium hy- droxide, more familiarly known as caus- tic soda or lye. Unless special means are taken to prevent the soda and the chlorine from coming in contact with one another, they will combine to form hypochlorite of soda, sometimes called soda bleach. Electric cells to produce this soda bleach are in use to-day in laundries, but they are inefficient and require frequent repairs. Special types of cells have therefore been designed to prevent the combina- tion of the soda and chlorine, so that the soda can be drawn off, as such, and the chlorine gas collected and combined with lime to form bleaching powder. Of from one compartment to another, or by means of an Archimedean screw which continually carries away the mercury amalgam, and brings fresh mercury to take its place. The Hargreaves-Bird, Allen-Moore, and Nelson cells differ in detail rather than in principle. In these cells the chlorine and soda are prevented from re- acting with one another by means of a diaphragm, which separates the anode compartment from the kathode. Into the inner, or anode, compartment, flows continuously a concentrated brine solu- tion, and on either side of the compart- ment is a diaphragm made of an asbes- tos compound, which becomes saturated with the brine and so permits passage of the current to the kathodes on the outer sides of the diaphragm. The soda pro- duced on the kathodes is washed down by steam which is injected into the outer compartments of the cell. In the Har- greaves-Bird cell, carbon dioxide is also injected into the kathode compartment, so that carbonate, instead of hydroxide of soda is produced. The advantage of the Castner-Kellner cell over the other FOUR STAGES IN THE MAKING OP THE AEMATURE ^ese cells the best known are the Cast- ner-Kellner, the Hargreaves-Bird, the Allen-Moore and the Nelson. In the Castner-Kellner cell, electrol- ysis takes place between an anode above and a kathode of mercury below. The chlorine gas rises to the top of the cell and is dravim off, while the sodium com- bines with the mercury to form sodium amalgam. The latter is decomposed by water to form caustic soda, the mercury being liberated and recovered for fur- ther use. The process is made continu- ous, either by giving the cell a rocking motion, which causes the mercury to flow three is the high purity and concentra- tion of soda produced, but owing to the large amount of mercury required the cost of installation is very high. The production of chlorates is carried out in cells similar in type to those de- scribed above, but no diaphragm is used, and the oxidation of the hypochlorite is promoted by the use of potassium chro- mate. Oxygen and hydrogen are pro- duced by electrolyzing acidulated water, cells fitted with iron electrodes being commonly employed for this purpose. Other chemical processes in which an electric current is employed are the pro-
