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CHEMICAL ACTION OF THE SOLAR RADIATIONS. 139 
Lieut, the Juminous power as distinguished from the chemical power, was the 
most active in producing the decomposition of carbonic acid by the leaves of 
plants* ; and these results were confirmed by my own researches published in 
the Reports of the BritishAssociationt. Although these influences upon living 
organisms are directly connected with the chemical influences of the solar 
rays, the phenomena being of a very complicated character, and requiring a 
very enlarged series of researches, it is thought advisable to confine attention 
mainly, in this Report, to the chemical changes produced upon disorganized 
matter. 
In 1803 Wedgwood, who was assisted by Sir Humphry Davy in some part of 
his experiments, published in the ‘Journal of the Royal Institution,’ vol. i. ‘An , 
account of a method of copying paintings upon glass, and of making profiles 
by the agency of light upon the nitrate of silver.’ In this communication 
we have the earliest indications of the photographic processes which have 
within a few years been brought to a great degree of perfection. ‘‘ Nothing,” 
says Davy, “‘ but a method of preventing the unshaded parts of the delineation 
from being coloured by exposure to the day, is wanting to render the process 
as useful as it is elegant.” 
An experiment on the dark rays of Ritter, by Dr. Young, included in his 
Bakerian Lecture} is a very important one. Dr. Young, after referring to the 
experiments of Ritter and Wollaston, goes on to say, “‘ In order to complete 
the comparison of their properties (the chemical rays) with those of visible 
light, I was desirous of examining the effect of their reflection from a thin plate 
of air, capable of producing the well-known rings of colours. For this pur- 
pose I formed an image of the rings, by means of the solar microscope, with 
the apparatus which I have described in the Journals of the Royal Institution ; 
and I threw this image on paper dipped ina solution of nitrate of silver, 
placed at the distance of about nine inches from the microscope. In the 
course of an hour, portions of three dark rings were very distinctly visible, 
much smaller than the brightest rings of the coloured image, and coinciding 
very nearly, in their dimensions, with the rings of violet light, that appeared 
upon the interposition of violet glass. I thought the dark rings were a little 
smaller than the violet rings, but the difference was not sufficiently great to 
be accurately ascertained : it might be as much as 5), or 7, of the diameters, 
but not greater. It is the less surprising that the difference should be so 
small, as the dimensions of the coloured rings do not by any means vary at 
the violet end of the spectrum so rapidly as at the red end. The experiment 
in its present state is sufficient to complete the analogy of the invisible with 
the visible rays, and to show that they are equally liable to the general law, 
which is the principal subject of this paper,’’ that is, the interference of light. 
M. B. G. Sage, in the ‘Journal de Physique, 1802,’ mentions a fact ob- 
served by him, that ‘‘ the realgar which is sublimated at the Solfaterra under 
the form of octahedral crystals, known under the name of ruby of arsenic, 
effloresces by the light ;” and that ordinary native realgar from Japan changes 
to orpiment by exposure to sunshine§. 
In 1806 Vogel exposed fat carefully protected from the influence of the air 
to light, and found that it became in a short time of a yellow colour, and ac- 
quired a high degree of rancidity. Vogel subsequently discovered that phos- 
phorus and ammonia exposed to the sun’s rays were rapidly converted into 
phosphureted hydrogen, and a black powder, phosphuret of ammonia. He 
* Philosophical Transactions, 1836. + Report of Seventeenth Meeting, 1847, p. 17» 
1 Experiments and Calculations relative to Physical Optics, Phil. Trans., 1804. 
§ Philosophical Magazine, vol. xiii. 
