» 
Κλ κεθατολλλ aka te 
CHEMICAL ACTION OF THE SOLAR RADIATIONS, 137 
On the present State of our Knowledge of the Chemical Action of the 
Solar Radiations. By Ropert Hunt. 
TuE present state of our knowledge of the phenomena of chemical changes 
produced by the influence of the solar radiations, is very imperfect. But 
though we have scarcely advanced beyond the threshold of this new line of 
research, we are enabled to contemplate a large number of striking facts sur- 
rounding the very entrance of this fresh field for experimental investigation. 
It was thought advisable to gather these facts, which have been hitherto 
scattered through numerous Transactions of the learned societies and scien- 
tific periodicals of these islands, of Europe and of America, into a Report, but 
which should show all that has hitherto been accomplished in this branch of 
inquiry. This task having been committed to my hands by the British Asso- 
ciation at the recommendation of the Committee of the Chemical Section, I 
have now much pleasure in submitting the result of my labours to the con- 
sideration of this Meeting. 
I find it necessary to state the progress of the investigation on those re- 
markable phenomena, of chemical changes produced by the sun’s rays, for 
the purpose of reviving a consideration of many very curious facts, which have 
been recorded, but which, from the circumstance that the attention of men 
of science was directed more earnestly into other channels, at the time of 
their publication, appear to have escaped attention. In stating, however, the 
earlier researches, I shall be as brief as possible, since many of them stand 
merely isolated facts which require new investigations to connect them-with 
the subject in the improved form which it now wears, under the more advan- 
tageous lights which have been thrown upon it by the refinements of modern 
science. 
We find, from time to time, in the writings of the elder chemists, faint in- 
dications that the changes produced by sunshine in many substances had not 
entirely escaped their attention ; but it is not until the commencement of the 
eighteenth century that we have an exact record of any observations of these 
phenomena. 
Petit in 1722 noticed that solutions of nitrate of potash and muriate of 
ammonia crystallized more readily in the light than they did in darkness*. 
Scheele about 1777 appears to have been led to an examination of the con- 
ditions under which nitrate of silver was blackened by solar influence; and 
with that refined system of research which distinguishes every inquiry of 
this Swedish chemist, he employed the prismatic spectrum, and observed for 
the first time, that the nitrate and chloride of silver were blackened by the 
rays at the blue, or most refrangible end, while no change was detected by 
him under the influence of the rays at the red or least refrangible end of the 
spectrum+. 
Senebier repeated these experiments, and he states that he found chloride 
of silver darkened in the violet ray in fifteen seconds to a shade which required 
the action of the red ray for twenty minutes}. He also experimented on the 
influence of light in bleaching wax. 
In the Philosophical Transactions for 1798 will be found a memoir by Count 
Rumford, entitled, ‘An Inquiry concerning the Chemical Properties that have 
* Sur la Végétation des Sels. Mém. de Paris, 1722. In 1788, Cuaprat published, 
‘Observations sur l’influence de lair et de la lumiére dans la végétation des sels.’ Mé- 
moires de l’Acad. Roy. des Sc. de Toulouse, vol. iii.; and in the Journal de Phys., vol. 
xxxiv. Diztin 1789. Duiz# deals with the same subject in a paper entitled, ‘ Sur la cris- 
tallisation des sels par l’action de Ja lumiére.’ 
T Scheele, Traité de l’Air et du Feu. $ Senebier sur la Lumiere, vol. iii. p. 199 
