Intelligence and Miscellaneous Articles, 547 



again precipitate the same silver solution in presence of starch, and 

 obtain a yellow precipitate. 



I have paid much attention to the subject of the fading of prints, 

 and had been forcibly struck by the anomaly above referred to. The 

 explanation of the gentlemen just mentioned seemed very satisfac- 

 tory ; but the question appeared to have too much importance to 

 pass it by without verification. The following results were ob- 

 tained. 



In a test-tube was placed a little boiled starch ; in a second tube, 

 some water. Two or three drops of a five per cent, solution of nitrate 

 of silver were added to each, and then hydrosulphate of ammonia. 

 Both solutions gave a yellowish-brown precipitate without the slight- 

 est difference in shade or colour. The precipitate afforded by hydro- 

 sulphate of ammonia could not here differ from that produced by 

 sulphuretted hydrogen, but to remove any doubt on this score, the 

 experiment was carefully repeated with sulphuretted hydrogen. 

 The same result precisely was obtained. As a further confirmation, 

 the experiment was varied by the substitution of another organic 

 substance, viz. collodion, instead of the starch. The result was pre- 

 cisely similar. 



The different result obtained by MM. Davanne and Girard can 

 perhaps be explained as follows. The sulphide of silver is a heavy 

 substance, and when thrown down from a simple watery solution it 

 quickly subsides. But solutions thickened with organic matter, such 

 as gum, starch, gelatine, &c, retain a precipitate for a long time 

 in a state of suspension, thus preserving the original yellowish- 

 brown appearance. If, therefore, we prepare solutions as above, 

 and pass HS through them, allowing them to subside, and examine 

 them only after a time, \*e shall find in the one case a black preci- 

 pitate below a colourless solution, in the other a brownish viscid 

 liquid. But if we watch the process from the outset, we shall see 

 that the reactions are chemically identical, and differ only in respect 

 to the mechanical suspension which takes place in the one case. 



The experiment which I here cite may throw some light on the 

 origin of the yellow colour. It appears that when the sulphide of 

 silver is in a state of very fine division, its colour is yellowish brown, 

 as may be easily ascertained by treating a very dilute solution (e. g. 

 5 o\> u * n ) °f n itrate of silver with hydrosulphate of ammonia. Many 

 substances are only black in consequence of excessive intensity of 

 colour. Lampblack, for example, in a state of excessively fine divi- 

 sion is yellowish brown. Ink diluted is purple. Claus has shown 

 that the intensely black hydrated sesquioxide of ruthenium is, when 

 very finely divided, green. 



I therefore conclude : 1st. That it is as yet not absolutely demon- 

 strated (although probable) that the current opinion, ascribing the 

 fading of pictures to the production of sulphide of silver, is correct. 

 2nd. That if this opinion be correct, there exists no evidence that 

 organic compounds have anything to do with the production of a 

 yellow colour, the tint of sulphide of silver found in their presence 

 and in their absence being quite the same shade of yellow-brown. — 

 Silliman's American Journal, May 1864. 



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