This wonderful development has been largely due to the intrinsic merits of the process and the reliability of the machines manufactured although it has been indirectly assisted by the excessive cost of hypochlorite during 1915-1916. Liquid chlorine machines are being used for the purification of water on the Western Front of the European battlefield. The outfit is a mobile one and consists of a rapid sand filter, liquid chlorine apparatus, a small storage tank and solution tanks. Owing to the limited contact period available a large dosage of chlorine is employed and the excess afterwards removed by the addition of a solution of sodium thiosulphate. _Chlorine Water._ Marshall[8] has proposed the use of chlorine water for the sterilisation of water for troops. The solution is contained in ampoules which are of two sizes, one for water carts and the other for water bottles of one quart capacity. The coefficient of solubility of chlorine, from 10°-41° C. is _C_ = 3.0361 - 0.04196_t_ + 0.0001107_t_^{2}; when _t_ = 10° C. 1 c.cm. of water absorbs 2.58 c.cms. of chlorine or 8.2 m.gr., a quantity sufficient to give a concentration of 1 p.p.m. in 8 litres of water. Marshall has stated that, when pure materials are used, chlorine water is stable but the author is unable to confirm this. A saturated solution of chlorine in distilled water lost over 50 per cent of its available chlorine content when stored for five days in the dark at 70° F. The chlorine present as hypochlorous acid increased slightly but the quantity never exceeded very small proportions. Chlorine solutions decompose in accordance with the equation, Cl_{2} + H_{2}O = 2HCl + O. Although chlorine water appears to be of little value because of its instability there appears to be no reason why chlorine hydrate should not be successfully employed. The hydrate was first prepared by Faraday[9] by passing chlorine into water surrounded by a freezing mixture. A thick yellow magma resulted from which the crystals of chlorine hydrate were separated by pressing between filter paper at 0° C. The hydrate prepared by Faraday was found to have the composition represented by the formula Cl·5H_{2}O but later investigators have shown that more concentrated hydrates can be prepared. Roozeboom[10] prepared a hydrate represented by the formula Cl·4H_{2}O and Forcrand[11] one containing only 3-1/2 molecules of water (Cl_{2}·7H_{2}O). Chlorine hydrate separates into chlorine gas and chlorine water at 9.6° C. in open vessels and at 28.7° C. in closed vessels. Pedler[12] has shown that when the ratio of Cl_{2}:H_{2}O is 1:64 or greater, the mixture of chlorine hydrate and water exhibits great stability and can be exposed to tropical sunlight for several months without decomposition. Cl_{2}·64H_{2}O contains 5.8 per cent of chlorine and about 8. c.cms. would be required to give a concentration of 1 p.p.m. in 110 Imp. gallons of water, the usual capacity of a military water cart. BIBLIOGRAPHY