PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 709 



A Substitute foe G^llodion. 

 M. Pcrsoz, jr., has recently devised a method for obtuiniiig a 

 material possessing the same characteristic qualities as collodion. 

 The new substance is produced by dissolving silk in a suitable 

 solvent, and then separating the latter by means of dyalysis. If 

 the film be of a certain degree of thickness it assumes, on dyeing, 

 a golden tint, but this, no doubt, would scarcely be perceptible in 

 a thin film, such as would be used in photography. The salt pre- 

 ferred by Persoz is chloride of zinc, which, when kept at a warm 

 temperature, readily dissolves the silk, but if not "warmed the silk 

 takes much longer time to dissolve. Before employing the chloride 

 of zinc, it is heated with a small quantity of the chloride of zinc 

 in order to neutralize any excess of acid in the chloride, and then 

 iiltei'cd through a piece of fine cambric to remove the superabund- 

 ant oxyd. To separate the chloride of zinc from the solution of 

 silk M. Persoz uses an apparatus for dyalysis which is a kind of 

 scive, made by means of a broad strip of gutta-percha bent roiind 

 and cemented in the form of a cylinder, at one end of which is a 

 disc of parchment to form the bottom. The apparatus is floated 

 upon a vessel of water, and the silk solution, previously diluted 

 with water to the consistency of collodion, is poured into it. 

 The chloride of zinc percolates through the moistened disc of 

 parchment, and mixes with the water in which the apparatus is 

 floating. In a few days the whole of the chloride of zinc will be 

 found to have separated from the silk solution, but the presence 

 of a slight quantity of the chloride of zinc in the material is of no 

 great consequence, as it merely gives rise to the formation, in the 

 sensitive film^ of a minute quantity of the chloride of silver. 

 Doubtless a dry film of of this substance would be quite insoluble 

 in water. Its employment in photography is very simple. It i& 

 first iodized by mixing with it an aqueous solution of iodide, and 

 then dried and sensitized. The exposure and the development of 

 the picture are conducted in the same manner as when ordinary 

 collodion is ustd. 



Ozone. 

 Dr. Daubeny, in a paper lately read before the London Chemi- 

 cal Society, has given the results of an extensive series of experi- 

 ments and meteorological observations made at Torquay and 

 Oxford. After describing the tests employed to indicate the pre- 

 sence of ozone and showing that the indications durinir the winter 

 mouths clearly pointed to the influence of the sea in augmenting 



