richards, McCaffrey, and bisbee. — calcic oxalate. 385 



oxalate, although necessary so far as the calcium is concerned, must in- 

 crease the amount of precipitated magnesium oxalate. But according to 

 •our original hypothesis, this excess is chiefly harmful when it is present 

 at the moment of precipitation, although it is not really needed to prevent 

 the solution of traces of calcic oxalate until the precipitation is practically 

 finished. Hence the oxalic acid also should be added gradually, or at 

 least in two portions, the first to supply enough oxalate to combine 

 with the bulk of the calcium, and the second to diminish the solubility of 

 the last traces. This plan was followed in the following work. 



Another point was as yet undecided, — the length of time needed for 

 the essentially complete separation of the calcium. Hence five determi- 

 nations similar to the above were made with solutions each capable of 

 yielding 0.1496 grams of lime and an equivalent quantity of magnesia. 

 The only variation in these experiments was in the time elapsing between 

 precipitation and filtration. 



The Effect of the Time between Precipitation and Filtration. 



The filtrates from the first two of these (Nos. 20 and 21) deposited 

 traces of calcic oxalate ou standing for two hours, but the others did 

 not deposit a trace in several days. From these five experiments two 

 conclusions may be drawn : first, that several hours are needed for a 

 separation of the last weighable traces of calcium, even when much 

 ammonic oxalate is used ; and secondly, that after the calcium has all 

 been precipitated, magnesium oxalate is absorbed by the precipitate at 

 a fairly constant although very slow rate, from a solution which will 

 of itself deposit no solid. The magnesic oxalate is slowly occluded by 

 vol. xxxvi. — 25 



