176 PROCEEDINGS OF THE AMERICAN ACADEMY. 



sulphate is possible by this method than from the other hydrates. Six 

 or seven crystallizations as the tetrahydrate give only the same degree 

 of purity as three or four crystallizations as the dekahydrate. 



The dekahydrates of sodium sulphate and chromate are isomorphous, 

 as already stated ; hence the sulphate must be previously removed by 

 crystallizing the dichromate in the manner already described. 



Crystallization of the pure chromate is best carried out at 5° in plat- 

 inum vessels ; and centrifugal drainage, as usual, greatly increases its 

 efficiency as a means of purification. After three or sometimes four 

 crystallizations the sodium chromate was found to give a transition 

 temperature which could not be raised by further crystallization. This 

 is, of course, the criterion to be used in deciding how long to continue 

 the purifying operations. The difficulties attending the purification of 

 this substance are no greater than those attending the purification of 

 sodium sulphate, provided that isomorphous substances are absent. 

 The mother liquors may be made to yield at least two more crops 

 of crystals ; their evaporation for this purpose is free from danger only 

 when the platinum vessels employed are so constructed as to prevent 

 the entrance of carbon dioxide. The yield of pure sodium chromate 

 may be nearly fifty per cent of the amount corresponding to the pure 

 sodium bichromate employed. Pure sodium chromate must be preserved 

 in a desiccator, containing a small amount of a solution of sodium hy- 

 droxide solution to absorb carbon dioxide. If the salt is to be kept for 

 any length of time it should be placed in platinumVessels. The tetrahy- 

 drate is the most convenient form in which to preserve the salt for long 

 periods, as it is permanent under usual conditions. 



The Hydrates of Sodium Chromate. 



The dekahydrate, Na2Cr04 • IOH2O, isomorphous with common 

 Glauber's salt, is so well known that description is unnecessary. It is 

 easily obtained in large crystals by inoculating a cold supersaturated 

 solution of the former salt with a crystal of the latter. In order to 

 avoid the suspicion of contamination with sulphate, inoculation was 

 carried out successively on several watch glasses, the first being touched 

 with sulphate, the second with chromate taken from the opposite side 

 of the first, and so on, the bulk of the material being inoculated with 

 pure chromate from the last watch glass. Whenever the temperature 

 of the laboratory rose above 20° this procedure was necessary, because 

 at this temperature all of the chromate in the laboratory suffered 

 change into the tetra- or hexahydrate, and so none remained where- 

 with to start the crystallization of a new specimen. Like sodium sul- 

 phate, the chromate has a wide range of metastability of supersaturation, 



