382 PROCEEDINGS OP THE AMERICAN ACADEMY. 



many precipitates require prolonged ignition in order to attain constant 

 weight. 



Over a year ago I was attracted by the observation that green chro- 

 mic sulphate is plentifully occluded by baric sulphate,* into beginning 

 a study of the subject of occlusion with the help of this phenomenon. 

 Chromium was selected instead of iron because its complex is more 

 stable and has been more thoroughly studied. The research was begun 

 by Mr. R. P. Gushing and is being continued by Mr. F. Bonnet. Since 

 the precipitated baric sulphate is green, one must infer immediately that 

 the green modification (or complex) is responsible for the occlusion. 

 The work is not yet ready for publication, but our data agree well with 

 those of Kiister and Thiel, while our explanation accords with that 

 given in the present paper. We are also studying aluminum sulphate, 

 which likewise seems to form a complex very considerably occluded 

 by baric sulphate ; although this fact does not seem to be generally 

 known. t 



This paper is intended to serve as an introduction to two other papers 

 upon specific cases of occlusion. The chief points upon which emphasis 

 is desired are the following : — 



(1) Occlusion from solutions differs from inclusion by being a more 

 intimate and a less mechanical mixture. 



(2) Occlusion differs from adsorption in concerning the whole mass of 

 the precipitate, and not merely its surface. Hence after a precipitate is 

 once formed occlusion takes place only with extreme slowness, because 

 of the rigidity of solids. New evidence of this will be given in one of 

 the following papers. 



(3) Different precipitates have almost as widely varying powers of 

 occluding other substances as different liquids have widely varying powers 

 of dissolving other substances. The pi'operty is no more to be predicted 

 than the power of dissolving. 



(4) The process of occlusion is probably the distribution of an electri- 

 cally neutral simple substance or complex between the solution itself 

 and the solid at the moment of its formation within the solution. Hence 

 any change in the substances present in the solution which tends to 

 diminish the concentration in the solution of the group under consider- 

 ation, tends to diminish the amount occluded. 



* This circumstance has been independently observed by Whitney, Jour. Am. 

 Chem. Soc. 21, 1075 (1899). 



t Compare Peckham, Jl. Am. Chem. Soc, 21, 772 (1899). 



