380 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



The next step was to test the effect of hydrochloric acid — or rather 

 of the coucentratioa of ionized hydrogen — upon the occlusion. Since 

 in this case from an acid solution all of the calcium could not be pre- 

 cipitated, a larger amount was used, but the concentration of the mag- 

 nesium was the same as before. 25 c.c. of the magnesium solution, 

 75 c.c. of the calcium solution, and 10 c.c. of normal hydrochloric acid 

 were diluted up to 200 c.c. To this solution, heated to boiling, were 

 added 27 c.c. of normal oxalic acid, to which had previously been added 

 10 c.c, of normal hydrochloric acid for the sake of diminishing its 

 dissociation. Over half of the calcium was precipitated in the form of 

 fine white crystals of the oxalate. It is well known that the substances 

 most capable of easy supersaturation are those which form the largest 

 crystals; and conditions which tend to promote the solubility of a pre- 

 cipitate in general tend to promote its ease of supersaturation. Hence 

 precipitates formed from solutions in which they are somewhat soluble, 

 are more crystalline than those formed from liquids in which they are 

 insoluble. Calcic oxalate is no exception to this general rule. The 

 easily handled precipitate was collected, thoroughly washed, ignited to 

 constant weight at a bright yellow heat, dissolved, reprecipitated, re- 

 ignited, and weighed again. The results below show how comparatively 

 .small was the occlusion of magnesium in this precipitate. 



