392 PROCEEDINGS OF THE AMERICAN ACADEMY. 



before. The resulting specimens of oxide weighed respectively 0.34G9 

 and 0.3474 gram, an average of 0.34715 gram, while the weights ob- 

 taiued by a similar doul)le precipitation in the absence of magnesium 

 averaged 0.34745 gram, or 0.3 milli<ifram more. This difference is too 

 small to have much signiticance, hence the conclusion is allowable that 

 as much of the lime as is practicable had been precipitated. 



It is clear from these experiments that the double precipitation usually 

 practised, while certainly eliminating the magnesium, inevitably involves 

 the loss of calcium. The single precipitation minimizes this last risk, 

 and by the compensation of two errors very small in themselves, gives 

 a result which is very near the truth. 



The conclusions attained in this paper may be summarized as 

 follows : — 



1. All those conditions which tend to diminish the amount of undis- 

 sociated magnesic oxalate in a solution tend to diminish the amount of 

 this impurity in calcic oxalate precipitated from the solution. 



2. Hence evidence is furnished supporting the hypothesis that occlu- 

 sion is a distribution of an undissociated substance between the liquid 

 and the " nascent " solid phase. 



3. Magnesic oxalate is precipitated upon the calcic oxalate after this 

 is deposited, although far more slowly than while it is being deposited. 

 For this reason the filtration should not be too long delayed. 



4. Calcic oxalate is sufficiently soluble in pure water to cause grave 

 inaccuracies in precise quantitative work. 



5. This solubility may be diminished, but not wholly prevented, by 

 an excess of ammonic oxalate in the wash water. 



6. By heeding all these relations, a reasonably precise separation of 

 magnesium and calcium may be made in a single precipitation. The 

 details are as follows : The majxnesium in the solution should be not 

 much stronger than fiftieth normal. About ten times the equivalent 

 amount of amraonic chloride, and enough oxalic acid to combine with 

 all the calcium, should be added to the mixed solution. It is well to 

 diminish the dissociation of the oxalic acid beforehand by the addition of 

 two or three times its equivalent amount of hydrochloric acid. To the 

 boiling mixture, colored with a drop of methyl orange, should be added 

 slowly a very dilute solution of ammonia, with continual stirring and 

 occasional pauses. The final stages of neutralization should not be 

 reached in less than half an hour. 



When the neutralization has been effected, a very large excess of 

 ammomc oxalate should be added, and the mixture should be allowed to 



