6 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
The disappearance of the darkened portions goes on with remarkable steadiness of 
progression from week to week and from month to month, till all is gone and the 
paper reduced to its original whiteness, or (if the ferrocyanate have been used in ex- 
cess) to a blue colour. 
19. By far the most remarkable fixing process with which I am acquainted, however, 
consists in washing over the picture with a weak solution of corrosive sublimate, 
and then laying it for a few moments in water. This at once and completely obli- 
terates the picture, reducing it to the state of perfectly white paper, on which the 
nicest examination (if the process be perfectly executed) can detect no trace, and in 
which it may be used for any other purpose, as drawing, writing, &c., being completely 
insensible to light. Nevertheless the picture, though invisible, is only dormant, and 
may be instantly revived in all its force by merely brushing it over with a solution of 
a neutral hyposulphite, after which however it remains as insensible as before to the 
action of light. And thus it may be successively obliterated and revived as often as 
we please. It hardly requires mention that the property in question furnishes a 
means of painting in mezzotinto, (i. e. of commencing on black paper and working in 
the lights,) as also a mode of secret writing and a variety of similar applications. 
20. The chromate (and probably also the arseniate) of silver is insensible to light, 
and in certain cases, therefore, where the colour is of little consequence, might be of 
use as a fixing material. 
21. There is a remark which ought not to be omitted in regard to this part of our 
subject, viz. that it makes a great difference, in respect of the injury done to a photo- 
graphic picture by the fixing process, whether that picture have been impressed by 
the long-continued action of a feeble light, or by the quick and vivid one of a bright 
sun. Even supposing the pictures originally of equal intensity, the half-tints are 
much less powerfully corroded or washed out in fixing, in the latter case than in the 
former. It is probable that other atmospheric relations than those which refer to the 
extinction of the merely luminous rays are concerned in this phenomenon. In an 
atmosphere so loaded with coal smoke as that of the neighbourhood of London, 
peculiarities of absorptive action may have place which rarely or never occur else- 
where. The tint of coal smoke is yellow (as may be seen in perfection in a London 
November fog), and more than one instance of the intense power and capricious 
singularities of very pale yellow media in their action on the chemical rays will come 
hereafter under our notice. In the locality from which this paper is dated, a light 
easterly wind brings with it abundant smoky haze from London, to which rural pre- 
judices assign the name of “ blight” and attribute an insect origin. On such occa- 
sions, when the sky has been otherwise cloudless, (comprising nearly half the poor 
allowance of sunshine of the last summer,) I have been continually at once annoyed 
and surprised, by the slowness of photographic action, and by the fugitive nature of 
its results under the process of fixing. 
