530 



NATURE 



{Sept. 12, 1878 



is coarse and has a tendency to subside, and which, 

 when poured upon a glass plate and dried, gives a film 

 transmitting white light ; whilst if we add it drop by drop 

 and shake it up between each addition, we find that the 

 bromide remains suspended for days, and that a film of 

 it transmits orange light. If we take the emulsion, how- 

 ever, as first described, and place it in a still, and bring 

 it to a state of ebullition, distilling over the ether and 

 alcohol solvents of the collodion, thus breaking up the 

 heavy particles of the suspended bromide, and then wash 

 the contents of the flask in water to get rid of all soluble 

 salts (which must necessarily exist owing to the double 

 decomposition of the soluble bromide and the silver 

 nitrate), we shall find, on redissolving the mass, after 

 washing with alcohol, that the film will transmit lavender 

 or a sky-blue light. On the other hand if the solvents 

 from the emulsion which transmits the orange light be 

 allowed to evaporate spontaneously and the solid residue 

 be then washed and redissolved, the film will still transmit 

 red light. It will thus be seen that silver bromide may 

 be secured in two states in one of which it principally 

 absorbs the blue and in the other the red. The chances are 

 that the former is sensitive to the less refrangible rays of 

 the spectrum, whilst the other is only sensitive to the more 

 refrangible rays. 



The emulsions, if made exactly as described above, 

 might or might not give a film which, after exposure to the 

 image formed in the camera, would develop without 

 blackening all over under the action of the developer, or, 

 in photographic parlance, might or might not develop 

 without fog. If the soluble bromide be in excess in the 

 emulsion, and be then washed out as described, the 

 sensitive films would be free from this evil, whereas if 

 the silver nitrate were in excess, this would not be the 

 case. The cause of this difference in behaviour has 

 lately been explained, though the means of correct- 

 ing the emulsion has long been known. The reasoning 

 will perhaps be rendered more clear if the experimental 

 proofs be recounted in the order in which they were 

 made. We must suppose that we have at hand a perfect 

 emulsion, a film of which will give a bright image 

 on development, after exposure in the camera. Let 

 half-a-dozen plates be prepared with such an emulsion 

 by simply flowing it over glass plates and allow- 

 ing the films to dry; and then let these be ex- 

 posed in the camera for the time necessary to give a 

 strong image on development. Let the plates be immersed 

 respectively after exposure in a weak solution of nitric acid, 

 of potassium permanganate, potassium bichromate, nitrous 

 acid, hydroxyl, iodine or bromine vapour, or be exposed 

 to the action of ozone, it will be found that the image 

 impressed by light will steadily refuse to develop, however 

 much it may be coaxed ; or again, if another half-dozen 

 plates be prepared and be exposed to light external to the 

 camera, we know if exposed to the lenticular image after 

 such treatment, that we might obtain an image on develop- 

 ment, but that it would be obliterated by the veil induced by 

 the preliminary exposure. If between the preliminary ex- 

 posure and exposure in the camera the plates be treated 

 with any of the above solutions or vapours, and be then 

 washed, we should find the impressions of light in the 

 camera would yield images perfectly free from the veil. 

 In other words the treatment of the film with any of these 

 solutions will destroy the effect of the action of light. Now 

 as we have already shown, the image is formed of silver 

 sub-bromide ; hence we may say that the treatment has 

 changed the sub-bromide to an undevelopable compound. 

 When exposed to iodine or bromine the sub-salt will 

 naturally become converted into the primitive salt, but 

 when we look at the nature of the other destructives, we 

 cannot but be struck with the fact that they are either 

 solvents of metallic silver or oxidising agents. In the 

 first case we may presume that the loose atom of the 

 silver of the sub-bromide (Ag2Br) is dissolved away and 



converted into some other form of silver, leaving behind 

 the half molecule of bromide, and in the other we may 

 presume that the sub-bromide is oxidised to form an oxy- 

 bromide of silver. 



With this fact as a starting-point, it appeared probable 

 that the elimination of a veil due to an emulsion ought to 

 be effected by the same agents as if the veil occurred 

 through the action of light. It was well known that in 

 order to get an emulsion perfectly free fiom this enemy 

 that chlorine, bromine, iodine, or some diad chloride or 

 bromide were necessary to be added to the washed 

 emulsion if the silver nitrate were in excess at first, and 

 that nitric acid had the same effect if added to the 

 emulsion with the silver nitrate. Here, then, seemed to 

 be the proof of what was wanted, but another link was 

 still required to make the reasoning complete. In making 

 an emulsion if the soluble bromide was in excess none of 

 these agencies were required. The question then arose 

 as to why this was the case. To clear this up a fair 

 hypothesis was taken, viz., that no soluble bromide was 

 absolutely free from contamination. If the bromide were 

 of the alkalis or some of the metals such as zinc, it was 

 probably contaminated with the oxide, whilst with other 

 diad metals it was probably of the lower form of bro- 

 mide. Thus cupric bromide was probably contaminated 

 with cuprous, and cobaltic with the cobaltous, though 

 in infinitesimally small quantities. Now the former im- 

 purity would cause the formation of silver oxide, and the 

 latter of silver sub-bromide (argentous bromide). Ex- 

 periment showed that the former would act as a nucleus 

 on which the metallic silver, reduced by development, 

 would be deposited, whilst the latter would have the same 

 composition as the latent image and thus induce the 

 objectionable veil. The same reasoning applied to the 

 chlorides, but the whole explanation was still incom- 

 plete. Experiment showed, however, that the order of 

 formation of the different compounds of silver was as 

 follows. Argentic bromide formed first, next argentic 

 chloride, then argentous bromide and chloride, and 

 finally the oxide. If, then, there was but little of the 

 impurity present in the soluble bromide used in forming 

 the emulsion, pure silver bromide would alone be formed,, 

 leaving the impurity in solution and in a state to be 

 washed out. The whole subject of the fog-giving proper- 

 ties of emulsion was thus cleared up, and the correction 

 necessary for it was apparent. 



All practical photographers are aware that in the 

 ordinary dry plate processes there is a deterioration of the 

 image if plates be kept a long time before exposure and 

 development, and if sufficiently long time elapse that the 

 image will almost refuse to develop at all. The question 

 arises why this obliteration of a developable photographic 

 image takes place ? We have seen how an image can be 

 destroyed artificially by the use of oxidising agents, and we 

 might naturally infer that the same destructive agency 

 might obliterate the image even when the oxidising agent is 

 merely ordinary air, and after considerable experimental 

 proof we are compelled to come to the conclusion that this 

 is the case, more especially when it is found that any 

 readily oxidisible matter, such as gallic acid, if applied in 

 solution and dried in contact with the sensitive film, 

 preserves the image for a longer period than if this pre- 

 caution be omitted. The oxidisable matter has to be 

 oxidised before the image itself is attacked, for we rnay 

 assume that the image itself is not as readily oxidi- 

 sable as such bodies as that mentioned, and many others. 

 The fact that the image can be oxidised and thus 

 destroyed, seems to disprove the once held opinion that 

 the undeveloped image was formed of metallic silver, a 

 body which will tarnish but not oxidise, the pure oxides 

 being unstable. 



As a sequence to the destruction of the photographic 

 image by oxidation, the hitherto unexplained results 

 which Draper obtained when photographing the spectrum 



