PUOCliEDlNGS OF SOCIETIES. 201) 



Sidebotham called especial atteution to the peculiarity of the form 

 of crystallization, and to the fact that an inorganic salt, in 

 contact with organic matter, should produce vegetable forms. 



The Secretary then read a paper by Mr. Petschler, describing 

 the plates and the process. 



Glass plates, Nos. 1, 2, and 3, were coated with collodion, on 

 the surface of which a hot mixture of gelatine and bichromate 

 of potash had been poured, then allowed to cool and to dry spon- 

 taneously. In a few hours the crystals began to form and ramify 

 themselves over the plate. The mixture was composed of three 

 parts of gelatine and water tv.euty grains to the ounce, to one part 

 of a saturated solution of bichromate of potash. 



Plate jN'o. 4, the same mixture spread hot without collodion. 

 On a corner of the plate the crystals have been dissolved out with 

 water, showing skeleton traces in the gelatine left behind. 



Plate No. 5 is covered with collodion and a solution of bi- 

 chromate without gelatine. 



Plate No. 6 was first covered with the mixture and then with 

 glycerine, but no crystallization took place. It was then dried 

 with strong heat, gelatine and bichromate poured over hot, and 

 then allowed to crystallize. 



Plate No. 7 prepared as No. 4, with gelatine and bichromate 

 without collodion. After the crystals were formed, the plate was 

 dipped into a solution of nitrate of silver, which changed the salt 

 into the red chromate of silver, insoluble in water, but soluble in 

 hyposulphite of soda, ammonia, &c. In one corner the crystals 

 were dissolved out, leaving their casts in the gelatine. 



Plate No. 8 was prepared as 1, 2, and 3, by collodion and the 

 mixture, and after the formation of the crystals changed into red 

 chromate of silver as No. 7. 



Plate No. 9 pi'epared as No. 6, with glycerine, dried with great 

 heat, then coated with the mixture and treated with nitrate of 

 silver as No. 7. 



The great variety and beauty of these forms of crystals could 

 with difficulty be represented by drawings. 



The author believes that no chemical combination takes place 

 between the salt and the gelatine, but that the latter acts simply 

 as a medium. The gelatine, when firm, retains a certain quantity 

 of moisture, which is favourable to crystallization ; but when the 

 moisture is driven olf by heat the crystallization is suspended. 



In the course of the conversation which ensued, the Chairman 

 referred to the ramified form in which the salts of some metals 

 were found naturally, in agate, slate, and even trap rock, where 

 the oxide of mn.nganese was frequently found to have assumed 

 similar forms. 



Mr. Mosley suggested that the arborescent forms might perhaps 

 arise from the tenuity of the solution, from the resistance of the 

 gelatine to allow of crystallization in the usual rhombic form, and 

 possibly to the subtle electrical or galvanic action supposed to be 



