150 



Mr. Rainey on the formation of 



[Mar. 30, 



part of the rhomboidal forms are seen to have disappeared, and to have 

 become changed into crystals of various sizes, and generally of a prismatic 

 shape ; after which the mixture is to be set aside for two or three days. If, 

 after the expiration of this time, all the rhomboidal forms have not disap- 

 peared and become replaced by prisms, fresh solution must be added until 

 such is the result. The size of the new crystals and their degree of com- 

 pleteness will be in proportion to the time employed in their formation. 

 (Fig. 5 is a representation of these crystals.) 



Fig. 5. 



This compound of the double oxalate and chloride can also be prepared 

 by keeping the rhomboidal form for three or four weeks in an atmosphere 

 saturated with aqueous vapour, and completely excluded from the external 

 air. By this mode of treatment these crystals in a few hours absorb 

 water, and begin to undergo a process of disintegration, and in two or three 

 days distinct well-defined prismatic crystals make their appearance. These 

 processes of disintegration and formation of new crystals continue until all 

 the rhombs are converted into prisms, which are contained in the water 

 absorbed from the humid air, in which they may be preserved if kept in a 

 well-stopped bottle, so as to prevent evaporation and the consequent re- 

 conversion of the prisms into rhombs. This form of double salt, like the 

 one from which it was prepared, is instantly decomposed, by the sudden 

 addition of a sufficiently large quantity of water, into oxalate of strontia 

 and chloride of strontium. It is not affected by exposure to the air, and it 

 is insoluble in absolute alcohol. 



The following is the formula deduced from the subjoined analysis by Mr. 

 Holmes : 



3C 2 Sr 2 4 .2SrC1.4H 2 O. 12Aq. 



