403 Prof. Penny on the Composition and Phosphorescence . 



it, when heated, a further quantity of the salt. In- plating sul- 

 phate of potash, a different mode of proceeding is adopted. The 

 soft sulphate is dissolved either in weak ley from kelp, or in 

 boiling water, until the liquor is about 44° of Twaddell. This is 

 evaporated to 48° or 50°, and run into coolers to crystallize. 

 When the crystals have fully formed and the mother-liquor has 

 completely cooled, the latter is siphoned off, and being saturated 

 with soft sulphate as before, it is again transferred to the coolers, 

 and a second ci'op of crystals deposited on the first. These ope- 

 rations are repeated several times, and in this M'ay from four to 

 six, or even seven, successive and cohering crops are obtained in 

 the form of a hard thick cake. The number of crops deposited 

 depends on the success of the process, the richness of the soft 

 sulphate, and the thickness of cake required. 



In plating directly from kelp, the leys, or liquors obtained 

 from steeping the kelp in water, are boiled down till the specific 

 gravity of the liquor indicates tliat sulphate of potash is beginning 

 to fall. The fires are then drawn, and after a brief reposurc, to 

 allow the floating impurities to subside, the liquor is run into 

 the cooler and crystallized. In a few days a crop of plate-sul- 

 phate is found adhering to the sides. The mothei'-liquor is 

 siphoned off, and a fresh quantity of concentrated kelp-ley being 

 run into the cooler, a second crop of crystals is deposited on the 

 sui'face of the first. A succession of crops is thus accumulated 

 until the plate or cake is considered of sufficient thickness. 



From this brief description of the process of plating, it is 

 manifest that the salt, whether made fi-om kelp-leys or soft sul- 

 phate, must be more or less charged with impurities. The ap- 

 pearance of the cakes, indeed, shows that it is far from being a 

 chemically pui-e substance ; and on examining it, the presence of 

 minute quantities of carbonate of soda, common salt, and inso- 

 luble matter, may be readily detected. The large and well- 

 defined crystals studded over the surface of the cakes are, how- 

 ever, nearly pure ; and when carefully selected, contain very 

 little foreign matter. 



II. Its Phosphorescence. 



The brilliant and sparkling light emitted from this salt at the 

 time of crystallizing has been frequently observed. It may be 

 witnessed by simply looking into a vat of liquor in process of 

 cooling and crystallizing, and particularly when the temperature 

 of the liquor is about 100° F. It is not a continuous luminosity 

 or phosphorescence of the crystalline mass, but an emission of 

 vivid sparks or scintillations as the crystals separate from the 

 liquor, analogous to the phosphorescence of arsenious acid in the 

 act of crystalhzing from the hydrochloric acid. 



