4 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
principal results, transposing only the second and third heads, so as to keep the sci- 
entific part of the subject as distinct as possible from the practical. And first then, 
I. Of fixing Photographs. 
13. As already observed, I find the surest and best fixing material to be a liquid 
hyposulphite, and of these I prefer that of soda. But in using it some precautions 
are necessary. The photograph must be first well washed by soaking it in water, to 
free it from all superabundant nitrate of silver, which is very apt to produce a sul- 
phuret on its first contact with the hyposulphite*. If the paper be prepared with the 
simple nitrate, the water must be distilled, since the smallest quantity of any muriatic 
salt present attacks the picture impressed on such paper with singular energy, and 
speedily obliterates it, unless very dark. A solution containing only a thousandth 
part of its weight of common salt, suffices to effect this in a few minutes in a picture 
of considerable strength. But if the paper be already muriated or prepared with other 
insoluble argentine compounds, this perfect purity of the water is not needed, and a 
small addition of salt is even beneficial, by removing the silver as fast as abstracted 
from the paper. 
14. This first washing greatly diminishes the sensitiveness of the photograph to 
further impressions of light, and if merely nitrated, destroys it entirely if the paper 
be thin. If otherwise, it may be considered as half fixed, and maybe preserved, and 
occasionally inspected in feeble lights, till convenient to fix it completely. To do 
this it must be thoroughly dried, and then brushed over very quickly with a flat 
camel-hair brush dipped in a saturated solution of the hyposulphite, first on the face, 
then on the back. This, having remained on it till the paper is completely penetrated 
with it, must be washed off with repeated and copious affusions of water, aided by a 
soft sponge, with a dabbing motion, often turning the picture, until the liquid comes 
off without the slightest sweetness. The photograph is then fixed, and may be dried 
and put by; but to make it quite secure it is best to repeat the process, and if the 
paper be thick, even a third time. 
15. The hyposulphite of soda and silver being liable to spontaneous decomposition, 
accompanied with separation of silver in the state of sulphuret, it is necessary to be 
very careful in washing away the very last traces of this salt, especially if it be in- 
tended to use the photograph for re-transfers, in which case a deposition of sulphuret 
within its pores is fatal, since it renders the paper unequally opake. It is for this 
reason we recommend to apply the hyposulphite concentrated and quickly; since if 
it be not in excess at every point of the paper, the deposit of sulphuret takes place at 
the first contact, and can never after be got rid of. 
16. Common salt used as a fixing material on paper, when it succeeds, probably 
acts as a solvent of the chloride or other argentine compound used. At least I find 
* See my papers above cited. 
