180
NOTES AND QUERIES.
[2d S. N 9., MAH. 1. '56.
sub-chloride of silver, as has usually been supposed ; that
this silver, by contact with the baths above mentioned, is
transformed into sulphide of silver, which is afterwards
modified by the vapours in the atmosphere. Experience
bas proved the correctness of this hypothesis.
To verify this analytically, MM, Davanne and Girard endeavoured to ascertain firstly, what was the state of the silver in the positive, fixed, and not toned, examining in- cidentally if there remained any hyposulphite of soda in the substance of the paper. Secondly, what was the state of the silver on a positive proof toned in the usual way, that is to say, by means of the hyposulphites charged with chloride of silver or acetic acid, those baths which photographers call old hyposulphites. The process which they employed to effect this analysis was very simple ; it consisted of impregnating the sheet of paper with a solu- tion of nitrate of potash and carbonate of soda, burning it and submitting the ashes to analysis ; after the calcina- tion, the silver remained in an insoluble state, whilst the chlorine and the sulphur were transformed into chloride and sulphate. The accuracy of this process was first veri- fied by burning a sheet of paper impregnated with chloride of silver, and determining the quantity of silver in the ash by chlorine, and the quantity of chlorine by means of silver, and weighing the ttyo precipitates of chloride, they were found to be identical. In the same wav the com- position of the ash of photographic paper was determined, with a view to further analyses.
To decide the first question, a sheet of paper, impreg- nated with chloride of silver, was completely blackened by exposure to the light, washed in fresh hyposulphite of soda, then in distilled water, and finall}' burned ; no trace of sulphate was found in the ash. The quantity of chlorine amounted to 0'002 grammes ; that of the silver to 0'124 grammes. It was evident from this, in the first place, that the fresh hyposulphite of soda had left no trace of sulphur, and in the next place, that the proportion of chlorine was so small compared with that of the silver, that it might be considered as an impurity in the paper ; the formula Ag. 2 Cl. would have required ten times as much, that is to say, 0'020 grammes. Several times repeated, this analysis always gave the same results. But before drawing a conclusion, MM. Davanne and Girard wished to give it a more palpable form. They prepared a considerable quantity of chloride of silver, and spread it in a capsule, and exposed it to the light of the sun for a whole day, stirring it continually; it was washed with hyposulphite of soda, then with distilled water, and the residue, fused with pure carbonate of soda, gave a button of metallic silver, but the flux did not contain any traces of chlorine. It must be added, that the surface of a photographic print is perfectly soluble in acetic acid, whilst the sub-chloride is considered insoluble.
From these experiments, it may be concluded that the
positive photographic image is formed by metallic silver,
" loride supposed.
and not by sub-chloride of silver, as has been hitherto
To determine, in the next place, what was the state of
the silver on the toned proofs, they analysed a cer-
tain number, on which the desired black tints had been
produced, by means of the ordinary toning baths (hypo-
sulphite of soda mixed with acetic acid, or salts of silver),
and not only was silver always found, but sulphur also ;
these two substances occur together almost in atomic
proportions, such as are required in the formula Ag. S.
This result is continually reproduced, and we may there-
fore conclude, that in the above-mentioned toning baths,
the silver with which the p;iper is covered is transformed
into sulphide, a reaction easy to comprehend when we
recollect that the hyposulphites are immediately decom-
posed by acetic acid, and, as experience has shown, these
salts mixed with a solution of nitrate of silver are trans-
formed almost instantaneously into sulphide of silver.
Proceeding from this to the study of prints that had suffered a change, they submitted to" analysis some that had been prepared several years ago, and of which the black colour had been transformed into yellow; these they had prepared themselves, and had washed them for several days with water, after coming from the toning bath ; and also others that they had directly sulphurised. In each case they found sulphur and silver, and, what was curious, the proportions were the same as in the black prints coming from the toning bath.
It seems to be clear, therefore, that in prints that have merely been fixed, analysis indicates only the presence of silver, while in those that have been toned, whether they are black or yellow, there is both sulphur and silver, and these substances only. It remained to be seen whether this sulphuration was really the cause of the destruction of the image. In order to determine this, they sulphurised properly fixed prints, both by the photographic process, a bath of hydrosulphuric acid, and a current of sulphuretted hydrogen, and in every instance where the sulphurised prints came in contact with moisture, the black tones rapidly disappeared and gave place to yellow, while those prints that had merely undergone the fixing process showed no change.
The conclusion which MM. Davanne and Girard de- duce from this is, that sulphuration is the cause of the toning, and, in the presence of moisture, causes the de- struction of the print. But that the employment of the salts of gold, which give rise to a totally different re- action, is productive of no mischief.
The question still remains to be investigated, why the black sulphide of silver becomes yellow in the presence of moisture. As there is no change in the proportion of the elements constituting it, we are forced to admit, either a hydration of the compound, or an isomeric modification, analogous to the red and black sulphides of mercury.
In conclusion, they state, that it is easy, when a print has become faded, to restore the black tones, which may be augmented or diminished at pleasure. All that is necessary is to immerse it for a few hours, and in the dark, in a bath containing two or three grammes of ter- chloride of gold to a litre of wateV, a double decomposi- tion takes place, and the gold is deposited in the place of the silver. The chloride of silver formed, is then re- moved by a weak solution of hyposulphite of soda and the print washed ; the picture will be found to be completely restored.
ta Mtnai'
" Myrrour of the Worlde " (2 nd S. i. 153.) As I possess a copy of the second edition of this work mentioned by you, a volume also of the greatest rarity, if MR. KENSINGTON would like to compare it with his imperfect copy of the first, it will give me much pleasure to show it him ; and I should also be gratified by a sight of the one in his possession. My address is No. 6. St. Mary's Place, West Brompton, and perhaps he will kindly communicate with me on the subject. The actual comparison of copies of these very early printed books is of great importance. J. O. HALLIWELL.
" The Wanderer and Traveller " (2 nd S. i. 94.) The author of The Wanderer and Traveller
