(564.) Ritter attributed to these rays a deowidizing qua- especially as regards the nature of the substances 
Chemical }; ; 
pape othe lity. Dr Wollaston, who also made experiments on employed. 
spectrum. the subject, and discovered the specific action of the But the great impulse given to this subject was ne : 
different rays on gum-guiacum, prudently suggested derived from the invention or discovery of the beau- photagell 
the more comprehensive term of chemical rays. Va-  tiful art of Puotocrapny. hy. 
rious other denominations have been proposed, which In 1802 Mr Thomas Wedgewood and Sir Hum- _(568.) 
need not be here dwelt upon; all that can be said is, phrey Davy succeeded in forming pictures of objects Wedge: 
(565.) 
922 
violet rays (when the sun’s light is decomposed by a 
prism), in the same manner as the invisible rays of 
heat were found by Sir Wm. Herschel to extend be- 
yond the visible red. Scheele had indeed previously 
noticed that the power of the sun’s light to decom- 
pose and blacken salts of silver increased rapidly 
from the red towards the violet ray ; and there is little 
doubt that Herschel’s discovery suggested that of 
Ritter, of the independent or non-luminous rays of 
the spectrum. 
that deoxidation does not represent the solar action 
completely. Whether it be really an independent 
principle in the sun’s rays which causes these effects, 
or merely light and its modifications, is, as in the 
corresponding case of heat, yet undecided. But it 
is remarkable that, as shown by M. E. Becquerel, 
the discontinuity of the luminous spectrum produc- 
ing “ Fraunhofer’s Lines” exists equally for the che- 
mical rays. 
Soon after Ritter’s first experiments, Dr Young 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI. 
the emerald transmits the same rays. Red glass 
stops most of the chemical rays, whilst the garnet 
transmits them. Though white glass has generally 
been considered very transparent for these rays, Mr 
Stokes has shown that it entirely stops those of the 
very highest refrangibility, which are readily trans- 
mitted by quartz. Sir John Herschel and Mr Hunt 
have made many interesting experiments with coloured 
media on the particular parts of the chemical spec- 
trum absorbed, but a great deal remains to be done, 
wood and 
laid on paper prepared with nitrate of silver, and in pavy, 
taking profiles (silhouettes) by means of shadows. 
They proposed to obtain similar effects by means of 
the camera obscura, but their paper was not suffi- 
ciently sensitive. The effectual bar to their proceed- 
ings was, however, this: that they could discover no 
means of fiving the shadows which they had ob- 
tained, or preventing the whole surface of the paper 
from being gradually blackened by exposure to light. 
In 1814 J. Nicernore Nierce, a retired proprie- 
(569.) 
They in- tor at Chalons sur Saone,? entered into a similar en- Nicephore 
percha proved the Interference of the obscure chemical aaa 
pits quiry, but by methods quite different. He employed“? 
and may beT@Y8 (Phil. T'rans., 1803), a conclusion successively 
polarized. 
(566.) In 1835 Mrs Somerville made some interesting minous impression was first brought into view by 
Mrs So- experiments on the permeability of different bodies a chemical process subsequent to exposure in the 
ee * to the chemical rays, similar to those of Melloni on camera, In 1825 Nicephore Niepce became associated 
ments. the heating rays (see chap. VI., § 8), and she found with Daguerre, who had previously been engaged in Daguerre. 
great and seemingly capricious variations in this re- the same research; they agreed to communicate the 
spect.' The account of them was addressed to Arago, results of their several experiments. The result, as 
and published in the proceedings of the French Aca- is well known, was the invention of the DacuerREo- 
demy. She found that green glass, coloured by cop- typz, not improperly called after Daguerre, who 
per, intercepted entirely the chemical rays, yet this was seems really to have worked it out almost entirely 
not due so much to its colour as to its other qualities for himself, after the death of Niepce in 1833; 
(which are also peculiar as regards radiant heat), for whilst so patient and determined was Daguerre in 
Mrs Mary 1 The maiden name of this accomplished lady was Mary Somerville ; she was born, I believe, at Jedburgh, and married first Mr 
claimed since by different physicists. Berard in 1812 
showed that these rays are polarized by reflection. 
Seebeck observed that the different rays impressed 
different colours upon salts of silver: and M. Edmond 
Becquerel long afterwards showed that the red and 
yellow rays, though incapable of commencing chemical 
action, in some instances have the power of continu- 
ing it when once excited by the more refrangible 
rays. 
the solar effect upon resinous bodies, and some 
at least of his pictures were executed on plates of 
pewter or of rolled silver. They were mostly copies 
of engravings, and the light parts corresponded to the 
lights of the originals. He, however, at length suc- 
ceeded in fixing impressions of views in the camera 
obscura, thoughin an imperfect manner, and after very 
long exposure. The pictures thus obtained had this 
in common with more perfect processes, that the lu- 
Somerville Greig, afterwards her relative Dr Somerville. She is known in British science not only as the able commentator of Laplace’s Méca- 
—magne- nique Celeste, but as the author of some ingenious and apparently convincing experiments on the magnetizing power of the violet 
tic action ray. Some anomaly, however, remains to be explained on this subject, as the result cannot always be obtained. Several years 
of light [?]. before Dr Faraday made his discovery of what he terms the “ magnetization of light” (see chapter vil., § 5), the writer of 
these pages supposed that the reaction of light and magnetism, observed by Morichini and Mrs Somerville, might be due to a la~ 
tent and casual polarization of the light which was not present in all the experiments ; and, in 
particular, he suspected that circu- 
larly polarized light might have a magnetic influence; but his experiments to this effect, in May 1836, were not successful, 
though he thinks them worth repeating, 
® Probably one of the MM. Niepce who, in the early part of the century, are said to have propelled a boat on the Saéne by a 
peculiar kind of air-engine, called pyreolophore. Delambre, Rapport Historique, 1810, p. 242. 
