378 PROCEEDINGS OP THE AMERICAN ACADEMY. 



It is curious also that they should have overlooked the following state- 

 ment in a paper published over ten years ago by Professor Jannasch and 

 myself. In speaking of the cause of the phenomenon under consideration, 

 we said : '' Es Itisst sich uumittelbar annehmen, dass das Eisen als eine 

 Doppelverbiudung mit Baryum als Baryum-Ferrisulphat gefiiUt wird, 

 welches wahrscheinlich eiu Molektil gebuudeues Wasser (sogen. Consti- 

 tutionswasser) enthalt."* Klister and Thiel have offered new arguments 

 which they consider as being in favor of the first part of this proposition, 

 without appearing to recognize that the proposition is not itself new. 

 They have moreover ascribed an arbitrary formula to the substancej 

 while we did not feel prepared either to assume its exact nature or to 

 explain the mechanism of its occlusion. 



Three years after this publication Schneider described some interesting 

 experiments in which evidence was given that the occlusion of the double 

 or complex compound is a variety of solid solution. f The idea was 

 new at the time, and did not meet with general favor ; but a knowledge 

 of the work would have saved Kiister and Thiel trouble. 



In 1894, 1 after the passing of two more years, Gladding published a 

 modification of Lunge's method of precipitating baric sulphate free from 

 iron, which is the most convenient of any which had appeared up to that 

 time. He precipitated the iron with ammonia, filtered without especial 

 washing, acidified the filtrate, added an excess of baric chloride, and then 

 collected on one filter both the main mass of the baric sulphate and the 

 small amount of the same substance which he obtained from the ferric 

 hydroxide. The chief difference between this method and the method of 

 Kiister and Thiel is the fact that the latter pointed out the uselessness 

 of filtering off the ferric hydroxide before adding the bai-ic chloride. It 

 is quite probable, however, that Kiister and Thiel did not know of Glad- 

 ding's work, and hence independently devised the whole process. 



Let us consider for a moment the nature and cause of this kind of 

 occlusion. In the first place it must be distinguished sharply from the 

 mechanical retention of mother-liquor in minute cells which all crystalline 

 precipitates exhibit. Even large crystals rarely contain cells of included 

 mother-liquor plentiful enough to increase their weight by more than the 

 fraction of a percent; and this impurity is of course chiefly water. It is 

 easy to test the magnitude of this inclusion by precipitating such a sub- 

 stance as calcic carbonate from a solution containing a large quantity of 



* Jannasch and Richards, J. prakt. Chem. [2], 39, 821 (1889). 



t E. A. Schneider, Zeitschr. phys. Chem., 10, 425 (1892). 



X T. S. Gladding, Jl. Am. Chem. Soc, 16, 398 (1894); 17, 347 (1897). 



