338 Method of preserving the Sensitiveness of Collodion Plates. 



nitrate, and at the sanae time to neutralize the invariable alka- 

 hnity of the commercial substance, we prefer to employ nitric 

 acid highly diluted, and added gradually to the magncsian salt, 

 previously dissolved in water, until a very faintly acid reaction 

 is communicated to blue litmus paper; any decided excess of 

 acid must be avoided, its presence being certainly antagonistic 

 to a high state of sensibility. 



The proportions wc have generally adopted are, — 



Nitrate of magnesia ... 4 ounces. 



Nitric acid quant, suf. 



Nitrate of silver 12 grains. 



Water 12 ounces. 



The silver salt must be added after the neutralization has been 

 performed, any precipitated chloride resulting from impurity 

 being removed by filtration. Before use it should be ascer- 

 tained that the solution really contaiiis silver, by transferring a 

 few drops of the clear fluid to a watch-glass, and mixing with 

 common salt, when a milky turbidity, however slight, will indi- 

 cate the presence of a sufficient amount of silver to sustain the 

 sensitive condition of the plate. 



The solution of nitrate of magnesia may, if preferred, be pre- 

 pared by double decomposition between sulphate of magnesia 

 and nitrate of baryta, mixing them in the proportion of their 

 chemical equivalents, and filtering off the insoluble sulphate of 

 baiyta. The only advantage in practising this method is the 

 certainty of obtaining a neutral solution when the pure crystal- 

 lized salts have been employed ; it will, however, be found im- 

 possible to exclude a slight excess of one or other of these salts ; 

 a small quantity of sulphate of magnesia was left in the solution 

 used in our experiments, but it did not appear to exert any in- 

 jurious influence. A small proportion of nitrate of silver must 

 as usual be added before use. 



The double nitrate of magnesia and ammonia we have also 

 employed with very good results. It was prepared by measuring 

 out two equal volumes of diluted nitric acid, saturating the one 

 with carbonate of magnesia, and the other with carbonate of 

 ammonia, and then mixing; the solution required the addition 

 of a few drops of very weak nitric acid to render it neutral, and 

 a small quantity of nitrate of silver. 



Nitrate of manganese, prepared either by dissolving the pre- 

 cipitated carbonate in dilute nitric acid, or by double decompo- 

 sition between equivalent quantities of crystallized sulphate of 

 manganese (MnO, S0^ + 4H0) and nitrate of baryta, gives, upon 



potassium, with a little starch paste, to the nitrate of magnesia dissolved 

 in water ; in the event of its containing nitrite, a blue colour or precipitate 

 will appear, according to the amount of this impurity that may be present. 



