235 
Plate No. 5 is covered with collodion and a solution of 
bichromate without gelatine. 
Plate No. 6 was first covered with the mixture and then 
with glycerine, hut no crystallisation took place. It was 
then dried with strong heat, gelatine and bichromate poured 
over hot, and then allowed to crystallise. 
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, hut 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, prepared 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 crystal- 
lisation ; but when the moisture is driven off by heat, the 
crystallisation 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 manganese w'as 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 crystallisation in the 
