350 Messrs. J. Spiller and W. Crookes on a Method for 



the former, seemed most convenient for our purpose on account 

 of their general deliquescent natui-e^ and for our first experiments 

 we selected the nitrates of lime, magnesia, and zinc, as most pi'O- 

 mising of success. These agents were at first tried in the above- 

 mentioned order ; but from a few preliminary trials we were 

 inclined to give the preference to the zinc salt, and having ob- 

 tained such satisfactory results with its use, are induced to com- 

 municate them at once rather than withhold them until our inves- 

 tigation of the other compounds shall have been completed. At 

 first we endeavoured to add the nitrate of zinc direct to the 

 exciting bath, but the quantity requii'ed to prevent so large an 

 amount of nitrate of silver from crystallizing out on the plate 

 rendered the solution too dense to work with. 



The following process can be recommended as having proved 

 perfectly successful in our hands; we do not doubt that with 

 more general use it may be considerably modified and improved, 

 but at present we have rather contented ourselves with establish- 

 ing the broad principle with such detail only as will suffice to 

 ensm-e good results, and to leave to a future period the consi- 

 deration of those minor points which only a long experience can 

 develope. 



The plate, coated with collodion (that which we emploj^ con- 

 tains iodide, bromide, and chloride of ammonium, in about equal 

 proportions), is made sensitive by immersion in the ordinary 

 solution of nitrate of silver (30 grains to the ounce), and after 

 remaining there for the usual time, is transferred to a second so- 

 lution of the following composition : — 



Nitrate of zinc (fused) ... 2 ounces. 



Nitrate of silver 35 grains. 



Water 6 ounces. 



The plate must be left in this bath until the zinc solution has 

 thoroughly penetrated the film (we have found five minutes 

 amply sufficient for this purpose, although a much longer time 

 is of no consequence) ; it should then be taken out, allowed to 

 drain upright on blotting-paper until all the surface moisture 

 has been absorbed (about half an hour), and then put by until 

 required. The nitrate of zinc, which is still retained on the plate, 

 is sufficient to keep it moist for any length of time, and we see 

 no theoretical or practical reason why its sensitiveness should not 

 be retained as long : expei'iments on this point are in progi'ess ; 

 at present, however, we have only subjected them to the trial of 

 about a week, although at theendofthatperiod they were hardly 

 deteriorated in any appreciable degree. It is not necessary that 

 the exposure in the camera should be immediately followed by 

 the development, as this latter process can be deferred to any 



