382 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



as the precipitation of amnionic magnesic pliosphate, require more time 

 for their completion, when much ammonic chloride is present.* 



If, as a matter of fact, the occlusion is proportional to the concentra- 

 tion of the uiidissociated magnesic oxalate, additional ammonic oxalate, 

 even in the presence of ammonic chloride, ought to inci-ease the weight 

 of the precipitate. The following experiments, similar to the last except 

 that in each case three grams instead of one of ammonic oxalate were 

 used, were made to test this point. 



The Effect of Excess of Ammonic Oxalate. 



Thus a trebling of the amount of oxalate present increased the error 

 by about a thii'd of its previous value. At first one is surprised that the 

 increase is not greater ; but it must be remembered that the oxalate was 

 added rather slowly, so that most of the precipitate was formed before 

 a large excess of oxalate was present. It is chiefly the concentration of 

 the magnesic oxahite present at the instant of precipitation, not the sub- 

 sequent amount in contact with the precipitate, which influences the 

 distribution. When the precipitate has once appeared as a solid, the 

 action must be confined to the surface ; for diffusion into solids is ex- 

 ceedingly slow because of their rigid structure. 



A number of analyses were made in the hope of combining all the 

 circumstances which tend toward complete separation, and of eliminating 

 all those which oppose it ; but yet further difficulties arose. It seemed 

 probable that by (frndval neutralization of an ncid solution the calcic 

 oxalate might be preci|)itated in an environment containing as little mag- 

 nesic oxalate as possible, and thus be as free as possible from this impur- 

 ity. The mode of procedure was as follows. To a mixture containing 

 in 200 c.c, 25 c.c. each of the calcic and magnesic solutions, were added 

 three grams of ammonic chloride, 1 6 grams of oxalic acid, and enough 



* Compare Ostwald, Scientific Foundations of Analytical Chemistry (Mac- 

 millan, 1895), p. 130. 



