Richards, McCaffrey, and bisbee. — calcic oxalate. 379 



diminished by the presence of an excess of either amnionic hydrate or 

 amnionic chloride. When acetic acid was present, too little precipitate 

 was obtained, and a very large excess of oxalate in alkaline solutions 

 gave too much precipitate. But Presenilis attacked the problem merely 

 in an empirical fashion ; the guiding hypothesis of the present day had 

 not yet been suggested. It did not occur to him to study the effect of 

 gradual precipitation in a strongly acid solution. 



Most writers of handbooks upon quantitative analysis have accepted 

 Fresenius's method of double precipitation, and the recent literature 

 upon the subject is unusually scanty. In a paper, of which we have 

 seen only an abstract,* Hefelmann calls attention to the necessity of 

 using dilute solutions in order to attain satisfactory results ; but other 

 important references to the subject in periodical literature could not be 

 found. A good method devised by H. P. Talbot, not to be found in the 

 periodicals, will be mentioned later. 



For our experiments calcic chloride and magnesic chloride or sulphate 

 were prepared separately in a state of great purity. By the repeated 

 crystallization of the nitrate, the calcium material was freed from its 

 usual impurities, and the carbonate was precipitated from this nitrate by 

 means of pure ammonic carbonate. Standard solutions of this material 

 in hydrochloric acid were made from time to time, and the concentration 

 of these solutions was determined with the utmost care by precipitation 

 with ammonic oxalate, according to the method to be described. Mag- 

 nesic chloride and sulphate were carefully purified by repeated crystalli- 

 zation, and experimental proof was obtained of the absence of calcium 

 from them. The magnesium solutions were always of a strength approx- 

 imately equivalent to the calcium. Ammonic chloride was made by 

 passing pure ammonia gas into freshly made hydrochloric acid; oxalic 

 acid was especially purified ; and all the ammonia used was freshly pre- 

 pared in platinum vessels. 



In the first place, the worst possible results were obtained in order to 

 show how great an improvement was possible by the successive intro- 

 duction of the modifications suggested above. To neutral solutions of 

 a mixture of 25 c.c. each of the magnesium and calcium solutions, made 

 up to 200 c.c, was added an excess of ammonic oxalate, but without the 

 addition of either ammonic chloride or acid. The two precipitates thus 

 formed were ignited at a very high temperature until constant in weight. 

 Below are recorded the results : — 



* Zeitschr. anorg. Chem. 18, 401 (1898). 



