184 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
proper point, besides which this gas also acts powerfully on the alcohol employed. 
To obviate these inconveniences, paper thoroughly impregnated with guaiacum by 
washing with the tincture, and drying in a gentle heat, was steeped in weak aqueous 
solution of chlorine, by which process it slowly acquired a beautiful and pure celes- 
tial blue colour. It is very sensitive, and may be conveniently used for copying en- 
gravings, &c., which it does with this singularity, that the picture penetrates the 
paper and appears on the back of very nearly the same intensity as on the face*. In- 
deed, if the picture be over sunned the back will exhibit a perfect impression, while 
the face is spoiled, which produces a very strange effect : exposed to the spectrum, 
the blue colour is converted to a pale reddish yellow in the region of the less refran- 
gible rays, and simply whitened in the more refrangible region. The action, when 
prolonged till the light seems to have no further influence, extends from — 12 - 4, cor- 
rected for semidiameter, to + 40, or thereabouts, where it dies away insensibly. The 
maximum of photographic action occurs at — 8 - 7, and some trace of a minimum is 
perceptible at + 1 P5. Photographs taken on this paper, or spectra impressed on it, 
are fugitive — lose much of their force and beauty in a few days, and at length vanish 
altogether. 
155. When paper is washed with a solution of guaiacum in soda it acquires a green 
colour, though the solution itself is brown. By inclining the paper and carrying the 
wash always from below upwards, a very even tint may be obtained. The excess of 
liquid being blotted off, aqueous solution of chlorine was poured over it (on a slope) 
till all the alkali was saturated, and the liquid ran off smelling strongly of chlorine. 
Thus was produced a paper (No. 1168.) very evenly tinted, and varying in colour 
from a deep, somewhat greenish, to a fine celestial blue, according to the strength of 
the solutions employed. It is very sensitive, and is attacked with especial energy by 
rays in the spectrum, ranging from — 1T4 to + 11*4 with a maximum at — 9 0, the 
type being as in fig. 3. 
156. When paper so prepared is exposed, wet, to a temperature of 212° Fahr., it is 
immediately discoloured, the green changing to a sere or brownish yellow. The same 
change is produced after some little time at a temperature of 190°, and still more 
slowly, though yet completely, at 180°. At 175° the discoloration is incomplete and 
very slow ; and below that temperature the colour is not affected. If the paper be 
perfectly dried in a temperature gradually raised to 212°, the discoloration requires 
a considerably higher temperature, ranging from 220° to 275°, according to the time 
of exposure, being very slow at the former limit and almost immediate at the latter. 
These changes are independent of the action of light, being produced under mercury. 
157. The destruction by heat of the green or blue colour superinduced on guaia- 
cum by the more refrangible rays of light, was noticed by Wollaston, and it would 
seem, on a consideration of his experiments and of those described in the last article, 
that nothing further is requisite for operating the change from the green or blue to 
* For another remarkable case of this kind see the Postscript to this paper. 
