RICHARDS, McCaffrey, and bisbee. — calcic oxalate. 381 



The average error attained only one-fourteenth of its previous magni- 

 tude. Thus the pi-esence of acid has a remarkable restraining effect u[)on 

 the occhision ; a tact in accordance witli tlie prediction of the theory. 



In the next place, the action of ammonic chloride was studied. A 

 neutral solution of 25 c.c. each of the calcium and magnesium solutions 

 was made up to 200 c.c. and treated with two grams of ammonic chloride 

 and the same amount of ammonic oxalate as in the first experiment. 

 Two trials of this process yieldetl the following results : — 



The Effect of Am.iionic Chloride. 



About 1.1 milligrams more of calcic oxide was recovered from each of 

 these solutions by the addition of much more ammonic oxalate. This 

 precipitate, which fell after some time, was redissolved and reprecipitated 

 before weighing in order to free it from magnesium. The total amount 

 of magnesic oxide in the first precipitate was thus about one per cent, 

 or somewhat more than in the precipitates from an acid solution. 



These experiments show that ammonic chloride diminishes very much 

 the tendency of the magnesic oxalate to be precipitated, but that it exerts 

 also a similar although much less considerable effect upon the calcic 

 oxalate. The more ammonic chloride is added, the greater concentra- 

 tion of oxalate ion is necessary completely to precipitate the calcium, but 

 the more effective is the retention of the magnesium in the solution. 

 The limit to the advisable amount of ammonic chloride depends upon the 

 subsequent method to be used for the determination of the magnesium ; 

 but for ordinary purposes an equivalent-concentration ten times as great 

 as that of the magnesium present should answer. 



Tliis action may be explained, as has already been stated, partly by 

 an effect which would be caused by any electrolyte and partly by the 

 additional formation of an unstable complex. The existence of this 

 complex is abundantly confirmed by the other reactions of mafjnesium in 

 the presence of a large amount of ammonic chloride. It is well known 

 that many of the common reactions fail, and tliat other reactions, such 



