FRACTIONAL CRYSTALLIZATION. The crystals deposited at different periods during the evaporation or cooling of a liquid complex are not alike in composition, the order in which the different components crystallize being de pendent not only on relative solubility but upon the proportions present. That is, while there is a certain so-called °eutectic° mixture from which the components will crystallize simul taneously, all other proportions on continued evaporation or cooling form only crystals of the compound which is present in excess of the eutectic proportions. Thus a solution of more than 23.5 per cent of common salt in water if gradually cooled deposits only crystals of salt, but if the percentage of salt is less than 23.5 the solution deposits only crystals of ice and in each case when the residual liquid contains just 23.5 per cent of salt, the water and salt solidify together. With three or more compounds sim ilar but more complex relations exist.
When the compounds in the liquid are closely similar in crystalline form and have similar chemical formula; ((mixed crystal0 form (see CHEMICAL CRYSTALLOGRAPHY) which con tain molecules of the different compounds built into the same crystal. In such a case, at suc cessive stage of crystallization the proportion of the more soluble compound is found to in crease and advantage of this is taken to concen trate radium and to separate the rare earths of the didymium group. The processes are ex tremely laborious and may involve hundreds and even thousands of recrystallizations and may be briefly described as follows, the num bers representing the evaporating vessels.
After obtaining the mixed material in solu tion in the desired solvent this solution is evap orated in vessel one until a crop of crystals of sufficient size has formed. The crystals are fil tered from one and placed in two and dissolved; the mother liquor is placed in three; both by evaporation yield new crystals differing in com position from those first obtained.
The contents of two and three are now fil tered and the crystals 'and mother liquor and placed in new vessels. Vessel four, crystals from two; vessel five, mother liquor from two and crystals from three; vessel six, mother liquor from three. These three are made to yield three new crops of crystals, and so on, the number of fractions steadily increasing and the end members of each series differing most.
Fractional crystallization has long been used as a means of purifying compounds and was formerly much used for extracting silver from lead under the name of the Pattinson process, in which the silver-bearing lead is melted, cooled slowly and crystals of lead lower in sil ver obtained and transferred to another kettle where they are remelted and recrystallized. The mother liquor from the first lot of crystals is higher in silver than the original lead and is again cooled for another crop of crystals. In this way there can be obtained, with a series of crystallizations, essentially pure lead at one end and rich lead carrying $400 to $500 of sil ver per ton at the other.
It is recognized by the geologists that frac tional crystallization is the most general cause of the great differences between rocks formed from the same fused magma and fractional crystallization undoubtedly also explains nu merous concentrated deposits of oxides and sulphides in these rocks. The great chemical sediments, common salt, gypsum, anhydrite and, sometimes, salts of potash and magnesium occur in essentially separate beds rather than inter mixed because of fractional crystallization dur ing the evaporation of some ancient sea.