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Chemistry. — „ The condition of substances insohihle in water 

 formed in gelatine^ — By prof. C. A. Lobry de Bruyn. 



The condition of matter called the colloid, is in many respects a 

 subject of interest. The physiologist, the physicist, the chemist take, 

 each in his department, an interest in the peculiar (jualities of the 

 colloids. For to a great extent the processes of life take place in 

 a colloidal medium or between colloids; the part taken by colloid 

 bodies in osmose is generally known ; every chemist knows a certain 

 number of examples in which some substances appear in a colloid 

 form, between liquid and solid-amorphous or as so-called colloidal 

 solutions. 



An important study on the qualities of colloidal mixtures or 

 solutions, especially in a physic-chemical direction, we already owe 

 for many years to Mr. van Bemmelen. In these important experi- 

 ments Mr. VAN Bemmelen has studied, besides other qualities, quan- 

 titatively the changes taking place in a colloidal mixture prepared 

 with water (a hydrogel) in varying the temperature or the relative 

 humidity of the surrounding atmosphere. 



The observations of which 1 beg leave to give a concise survey, 

 do not refer in the first place to changes of a colloid mixture itself, 

 but to the influence exercised by a hydrogel on the physical condi- 

 tion of amorphous, insoluble substances, created in a hydrogel as a 

 medium. These observations are partly the result of former expe- 

 rience gained in the daily practice of the laboratory, as the non- 

 appearance of precipitates if in a qualitative analysis, say on metals, 

 there is an admixtui'e of substances like gum ; the non-precipitating 

 of chromate of silver in a gelatine solution a. s. o. Such observations 

 are in my opinion not indicated with sufficient accuracy by stating 

 that the substances formed remain in suspension. 



Experiences like those just mentioned were recalled to my memory 

 by a communication of Dr. Ernst Cohen in a meeting of the 

 „Amsterdamsch Genootschap." Mr. Gaedicke had asserted in a 

 photographic journal that if equivalent qualities of AgNOs and KBr 

 (somewhat more of the latter) are mixed in a gelatine solution of 

 5%, these two salts are only partially transformed, while this reaction 

 is complete in aqueous solution in consequence of the excessively 

 small solubility of Ag Br. This assertion of Gaedicke will appear 

 strange to no one who performs this reaction; he will namely not 

 see a precipitate of Ag Br forming itself, but only a mild opal- 

 escence ; while in aqueous solutions he will notice a strong troubling 

 arising. Gaedicke rested his assertion on the observation (proved to 



