THE WOMBAT. 6l 



change that took place might be due mainly if not entirely to 

 the effect of the alum or lead salt in the manner just described, 

 and not to the deposition of gold ; in other words the print 

 would seem to he gold-toned, whereas little or no gold was 

 actually present. Examination shows that with acidified 

 hypo., or hypo, mixed with alum, or neutral hypo containing 

 a lead salt, the toned image contained a notable quantity of 

 silver sulphide. The quantity, however, is not such as to 

 indicate that the whole of the image has been converted into 

 sulphide, nor as to make it permissible to attribute the change 

 of colour to " sulphurisation." The greater part of the image 

 still consists of the silvered compounds as altered or reduced 

 by light. It was observed by Just, and confirmed by Valenta, 

 that the paper itself obstinately retains small quantities of 

 lead from baths containing lead salts. Attention has recently 

 also been drawn to the fact by Mr. Baldock at one of the 

 London societies. The fact has been known for some time, 

 and the attention of chemists was called to it several years 

 ago by Mr. L. T. O'Shea. The specimens exhibited show 

 very clearly that pure paper will take up lead from a one per 

 cent solution of lead acetate with lead nitrate and retain it 

 even after prolonged washing. It is also clear that, in the 

 presence of hypo. (20 per cent.), the quantity of lead retained 

 -is much smaller, and that it is still further reduced by a second 

 treatment with hypo, solution containing no lead. Even in 

 the latter case some lead is still retained, and the quantity is 

 quite sufficient to cause discoloration of the paper in course of 

 time. 



The general conclusion, therefore, is that the use of lead 

 salts in a combined toning and fixing bath should be avoided 

 because of the danger of the lead being retained by the paper 

 in spite of thorough washing, and that the use of lead salts> 

 of alum, or of an acid should be avoided because any of them 

 will cause a print to acquire a colour indistinguishable from 

 that produced by gold, even though no gold at all has been 

 deposited. It is obvious that in this case the probable per- 

 manence of the prints is much lower than if they were pro- 

 perly toned with gold, and the observations described throw 

 light upon some previously puzzling cases of the fading of 

 gelatino-chloride prints. 



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