102 Mr. G. S. Cundell on the Gallo-nitrale of Silver. 



such have been employed, after a certain time (longer or 

 shorter, according to the strength of the solutions), on deve- 

 loping an impression in the usual manner, it will be found 

 that the whole of the surface, whether acted upon by light or 

 not, will become dark and discoloured ; and such prepara- 

 tions seem therefore to be of little or no value in photography. 



By adding to them, however, a little acetic acid, the ten- 

 dency to spontaneous decomposition is immediately lessened; 

 and by combining the three constituents in due proportion, a 

 " gallo-nitrate " may be formed of any degree of stability (or, 

 inversely, of any degree of sensibility) required. 



There are, no doubt, many proportions in which the ingre- 

 dients may be united with good effect; but, for general use, no 

 proportions perhaps are better than the original ones given 

 by the inventor. It is to be observed however of his formula, 

 that it is adapted to certain external conditions, and is not 

 equally suitable where these are different. It is not equally 

 suited to every climate or temperature of the air ; and although 

 well-adapted to excite paper for immediate use, it is perhaps 

 less so when the paper is intended to be kept : hence the cause 

 of many miscarriages in its use, and the necessity for some- 

 thing more than a mere formula for its preparation. 



If the acetic acid be deficient (which is frequently the case 

 from its uncertain strength), the spontaneous decomposition 

 already noticed will take place with more or less rapidity ; 

 and, unless the paper be used without delay, the impression 

 will be lost in the general darkening of the surface. 



On the other hand, if the acetic acid be in excess, there will 

 be less tendency to decomposition ; the paper will also be less 

 sensitive, but it will much better preserve its whiteness in the 

 excited state, and may be used successfully after many hours. 



By thus tempering the compound with acetic acid, accord- 

 ing to circumstances, it will be found that its action may in a 

 great degree be brought under control. 



If the gallo-nitrate of silver of Mr. Talbot be applied to 

 iodized paper, an action commences, which, unless very quickly 

 arrested, instead of merely exciting the surface and preparing 

 it to receive an impression, will speedily darken and render it 

 useless. It is most probably on that account that the paper is 

 directed to be "dipped into water;" the effect of which is to 

 arrest the action by diluting the gallo-nitrate, and by remo- 

 ving, perhaps, nine-tenths of it from the paper. The extent 

 however to which the destructive action may have gone, and 

 to which the gallo-nitrate may thus have been diluted or re- 

 moved, is all to the last degree uncertain ; and it seems a 

 more satisfactory proceeding to apply the gallo-nitrate in the 



