PROCEEDINGS OP Till: AMERICAN ACADEMY, 

 manj precipitates require prolonged ignition in order to attain constant 



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

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

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

 Chromium \\a^ selected instead oi iron because its complex is n 



.-talilc and has been more thoroughly >t u<l i«<l. The research was begun 

 by -Mi-. R. P. Cushing and i~ being continued by Mr. I-'. Bonnet Since 

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

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

 1 lie work is not yel ready for publication, bul our data agree well with 

 those <n Custer ami 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 he generally 

 known.t 



'I'li:- paper is intended to serve as an introduction to two other papers 

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

 is desired are the following: — 



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

 intimate and a less mechanical mixture. 



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

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

 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 power-, 

 of dissolving other sul»tances. The property i^ no more to he predicted 

 than the power of dissolving. 



I i The process of occlusion is probably the distribution of an electri- 

 cally neutral simple Bubstance or complex between the solution itsell 

 and the solid at the moment of its formation within the Bolution. 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. tend- to diminish the amount occluded. 



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

 Chcni. Soc, 21. in;-'. | 18 



t Compare Peckham, Jl. Am. Cbem. Boa, 21. 77:.' (18 



