AND ON THE ACTION OF LIGHT ON BODIES. 429 
are the blue, green and violet. They colour the red paper blueish- 
green, the violet rays even blue. If the paper has become green 
by exposure to daylight, and it is then laid in the sun under 
cover of red and yellow glasses, it acquires a bright red, some- 
what brownish tinge. Red and yellow therefore act on guaiacum 
which has been modified by the blue, green and violet rays, just 
as green and yellow on the blackened iodide of silver. And ac- 
cordingly one might expect that after the sun had coloured the 
red paper green, it would make it appear reddish if its action 
were continued, and such is found to be the case. The fact may 
be confirmed in a very short time if there be strong sunlight ; 
in the camera obscura, however, I obtained the image only after 
twenty-four hours, and even then the finer details were not very 
visible. 
I will now endeavour to prove my second statement, viz. that 
the action of light does not necessarily consist in the separation 
of two chemically-combined bodies. This part of my supposi- 
tion follows indeed from the discovery of Daguerre, and still 
more from that of Becquerel, according to which light produces 
changes in iodide and bromide of silver (the latter substance was 
experimented on by Becquerel), which are exhibited, it is true, 
by the mercurial vapours and red glass, but are otherwise quite 
invisible, and thus the assumption that iodine is hereby sepa- 
rated from silver is completely refuted. I shall hereafter de- 
scribe phenomena which teach us to produce effects similar to 
those of light in these experiments in a quite different manner, 
€.g. upon a pure surface of silver, where there can be no possi- 
bility of a chemical action. 
This refers solely to the primary modifications which light 
produces on the surface. Its continuous action is exhibited 
externally as a blackening of the iodide or other compound of 
silver, and this modification is no doubt considerable ; but it still 
does not admit of the assumption that light really does produce 
chemical decomposition. In order not to be misunderstood, I 
will remark that the other chemical effects of light, as.for in- 
stance on chlorine and hydrogen, on oxide of gold, some salts, 
nitric acid, &c. &c., do not interest me, for,—lst, many of these 
chemical effects probably deserve and indeed require an entirely 
New investigation in the present state of the science; and 2ndly, 
the principal point here is to prove the absence of chemical 
changes in one single well-known case that can be easily ex- 
