112 Methods of preserving the Sensitiveness of Collodion PlateS* 



riments necessarily require a more lengthened period than we 

 Lave at present been able to give, but as long as they have yet 

 been kept (upwards of three weeks), there has been no appear- 

 ance of deterioration. 



Before the development, we find it advisable to moisten the 

 Collodion film by immersion in the silver-bath for about half a 

 minute, as otherwise the pyrogaTlie acid or iron solution would 

 not flow evenly over the plate. The fixing, &c. is of course con- 

 ducted as usual. 



It will be as well to draw attention to a few points which, 

 although not absolutely essential, may possibly be found useful 

 in practice. The glass plates should be cleaned with more care 

 than is necessary when they are to be used immediately; we 

 have found strong nitric acid applied with a tooth-brush most 

 convenient. With regard to the collodion, we have tried very 

 many different samples, and with tolerably uniform success. 

 The greater number of our experiments have been made with a 

 tolerably thick collodion, the alcohol and aether of which were in 

 the proportion of 1 : 2, made sensitive with four grains of iodide 

 and half a grain of bromide of ammonium to the fluid ounce. 

 We have also employed a collodion containing iodide and bro- 

 mide of cadmium with good success. 



Of the 30-grain silver solution for exciting the plate we have 

 only to recommend the use of acetic instead of nitric acid, to 

 give the bath that faintly acid reaction which is by some opera- 

 tors considered desii'able. 



There are one or two circumstances to be attended to in the 

 preparation of the magnesia-bath. Commercial fused nitrate of 

 magnesia is very liable to contain chlorine, and also to have an 

 alkaline reaction on account of the fusion being carried too far. 

 Of course the quantities of acetic acid and nitrate of silver given 

 in the formula for the bath arc on the supposition that the 

 nitrate of magnesia is pure ; if this be not the case, it should be 

 rendered perfectly neutral with acetic acid, the chlorine exactly 

 precipitated with nitrate of silver, and then the proper amounts 

 of acid and silver added. However, if the impurities are very 

 considerable, it will be safer to reject the salt at once. This 

 bath will keep in good order for a long time ; the only point to 

 be attended to is to drain the plates slightly after coming from 

 the silver-bath, and, if necessary, to remove the liquid from the 

 back with blotting-paper, so as to introduce as little silver as 

 possible into the nitrate of magnesia. A solution of one grain 

 of silver to the ounce is quite sufficient to keep the plates sen- 

 sitive ; and when the strength rises, as it will in time, to above 

 a certain limit, the slight evaporation that always takes place 

 will render the silver solution sufficiently strong to dissolve off 



