ON ARTIFICIAL REFRIGERATION. 971 



sively, in the place of the aqueous sokitioii of glycerine, as an uncongeal- 

 able liquid in connection with ice-machines. 



The double chloride of potash and magnesium, washed with half its 

 weight of cold water, yields three-quarters of its potash in the form of 

 chloride of pdtassium ; and the remaining quarter, still held in solution 

 in the water used in this final operation, is retiu-ned to the boiler. The 

 commissioners of the United States who reported on this process in 18G7 

 remarked that "the separation of potash from sea- water, thus effected, 

 is one of the most important and valuable residts which science has, in 

 modern times, contributed to the industrial arts. Though potash is the 

 most useful of the alkahes, the natural sources from which it is possible 

 to obtain it economically are verj- few in number. Hitherto the sup- 

 ply has been chiefly derived from the ashes of land-plants, from which 

 it is separated by lixiviation. This resource, wliich continually grows 

 more precarious as civilization advances and as forests disappear, is des- 

 tined, doubtless, to give way to the process just described, and which 

 has already been for a number of years in active and successful opera- 

 tion." 



Messrs. Merle have applied the treatment above described to mother- 

 waters amounting to 100,000 cubic metres ])er annum, with an annual 

 product of 4,000 tons of anhydrous suljihate of soda, 1,000 tons of cldo- 

 ride of jjotassium, and 12,000 tons of refined table-salt. 



The waters leaving the refrigerators do not form incrustations in the 

 boilers, owing to the almost comijlete decomposition of the sulphate of 

 magnesia during refrigeration, the removal of sulphuric acid, in the sul- 

 phate of soda, and the increase in the quantity of magnesium chloride. 



By a process of downward freezing, such as I have described, pure ice 

 forms on the cooled metal surface and the salt-crystal deposit. Pro- 

 fessor Guthrie has in this way obtained ice containing only 0.4052 of 

 solid residue per cent. He experimented with sea-water from Dover, 

 having, after filtration at 760"'™, the boiling-point of lOO^.G C, while the 

 temperature of its vapor was 100^.2. This sea-water began to freeze at 

 2° C, for two hours the percentage of solid residue was C.5786. A 

 large beaker of this sea- water was cooled to 0^ C. A tin vessel was sup- 

 ported inside the beaker, so that its bottom just touched the surface of 

 the water, and a freezing-mixture was placed in the tin vessel. When 

 about one one-hundredth of the whole had solidified, the solid was 

 removed and divided into two parts : one was allowed to melt, and its 

 percentage of solid matter was determined as above; the other was broken, 

 u}) and frequently pressed between linen and flannel in a screw-press, 

 being allowed to melt as little as x)ossible. The percentage of solid matter 

 in this was also determined. The following numbers show the result of 

 this examination : 



Per cent, at lOO" 

 of solid residue. 



Sea- water G. 5786 



Frozen sea-water 5. 4209 



Frozen and pressed sea- water 0. 4925 



