Proceedings of the Polytechnic Association. S6?> 



and moisture would inidoubtedly have their effect. A pliotographer 

 of considerable eminence and large experience had recently told him 

 that he fonnd that negatives wrapped in paper never cracked, while simi- 

 lar negatives pnt away in plate boxes did so. He believed that where 

 the collodion was made thick, with a view to secure dense negatives, 

 the tendency to crack was increased, the thickness of the film affect- 

 ing, of course, its contractibility, and this might sometimes account 

 for some negatives produced with the same materials, and apparently 

 under the same circumstances, cracking, while others remained perfect. 



Mr. Blanchard said as Mrs. Cameron resided at Freshwater, her 

 residence would be subjected sometimes to winds bringing much 

 saline matter, and consequent moisture, and it was to this, he had no 

 doubt, the cracks were due. The best mode of preventing the effect 

 of damp was to pack the negatives in paper instead of keeping 

 them in plate boxes. Referring to Mrs. Cameron's negatives, the 

 appearance justified her impression that it was the collodion only, 

 and not the varnish, which was cracked. This he would demonstrate 

 by rubbing the surface with a little soot. It would be seen that bj 

 this operation he had once filled the cracks with soot, and made the 

 negative capable of being printed without showing cracks. 



After Mrs. Cameron had expressed thanks for this hint, Mr. 

 Hooper suggested that one of the causes of cracking was moisture 

 on the plate from the breath of the operator, which condensed on 

 large plates during the process of coating with collodion. 



Mr. Dallmeyer pointed out that some kinds of glass underwent a 

 change, technically called " sweating," a certain exudation appeared 

 on the surface, caused by excess of alkali in the manufacture. It 

 was impossible to wipe such glass clean, and the presence of this exu- 

 dation, causing dampness, might be the probable cause of cracking. 

 Glass in this condition might be cleaned by the application of dilute 

 sulphuric acid, one part of acid to four parts of water. 



Mr. Hart concurred in the idea that the cracking was duo to mois- 

 ture, and mentioned that, owing to the bursting of a steam-pipe in 

 his establishment, the walls had become damp on one occasion, when 

 the films of the negatives in a cupboard all rose in ridges, finally 

 causing cracks. 



Mr. Howard expressed the opinion that moisture M-as the cause of 

 the difficulty. He attached considerable importance to warming the 

 plate thoroughly before varnishing, and, after varnishing, to drive off 

 all traces of alcohol. 



