CHANGES PRODUCED BY THE SOLAR RAYS. 145 
the roots of jalap and ipecacuanha become pale, and, parti- 
cularly the former, are rendered comparatively inert. The 
tinctures of cascarilla, catechu, kino, and frequently cincho- 
na, deposit a considerable portion of the vegetable matter 
held in solution by the spirit on the side of the bottle expos- 
ed to the most light ; the same result takes place with some 
others. In chemical preparations, many very remarkable 
changes take place, which need scarcely be particularized. 
The changes in oxide of silver, in oxide of mercury, and in 
nitric acid, have frequently been described. The effect of 
even diffused light on hydrocyanic acid is familiar to every 
Druggist, and there can be but few who have not observed 
alterations in the character of the peroxide of mercury (red 
precipitate) and the chloride of mercury (calomel) by the 
action of the sun ? s rays. The bleaching of the fixed oils 
and of wax need scarcely be mentioned, but the influence of 
light upon the resins, with the exception of guaiacum, was 
never pointed out. until the publications of M. Niepce's 
photographic researches. 
I am not aware that any investigations have been made 
into any of these changes : I am therefore induced to trou- 
ble you with a few remarks, the results of experiments 
which have extended over some years, and which, although 
far from complete as a scientific investigation, have the 
merit of some practical and really important applications. 
I shall not detail my experiments, which have been very 
numerous and frequently repeated, but, for the present at 
least, content myself with giving the general results of these 
researches. 
It will be necessary, however, to describe the mode 
adopted by me in this examination, and the various condi- 
tions under which the experiments were made. Having 
selected the bright green powder of the leaves of the medi- 
cinal plants, carefully dried in the dark, and powdered by 
myself, it was placed in dry and warm glass tubes, and 
these were then hermetically sealed. Some of these tubes 
13* 
