On the Simultaneous Emission and Absorption of Rays. 193 
tion), there will then be no longer any difficulty in referring 
these results, with others of the same class (e.g. the several 
varieties of gold prepared and examined by Professor Faraday*), 
to a series all of which are capable of similar explanation. 
I subjoin, in the form of propositions, a statement of the 
results arrived at; they appear to me to have been fully sub- 
stantiated by the foregoing experimental considerations. And I 
will remark, in conclusion, that the hypothesis believed to be 
supported by the facts now communicated is in conformity with 
the previous results of Dr. Guthrie, MM. Girard and Davanne, 
and generally also with those of M. Van Monkhoven, and will 
consequently be to a certain extent opposed to the views advanced 
by Messrs. Hadow, Hardwich, Llewellyn, and Maskelyne, in 
their joint report upon this subject recently presented to the 
meeting of the British Association. 
Propositions. 
Ist. That chloride of silver, when decomposed by light, is 
separated into its elements. 
2nd. That this change does not usually extend to the whole 
bulk of the material operated upon, on account of the opacity of 
the darkened product mechanically protecting a certain portion 
of unaltered chloride of silver from the action of the light. 
8rd. That the degree and rapidity of reduction is influenced 
by the state of division of the particles, and by the presence of 
agents capable of absorbing the chlorine when liberated from its 
combination with silver. 
Chemical Department, Royal Arsenal, 
Woolwich, February 17, 1860. 
XXV. On the Simultaneous Emission and Absorption of Rays of 
the same definite Refrangibility ; being a translation of a portion 
of a paper by M. Lion Foucavutt, and of a paper by Professor 
Kircunorr. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
ei years ago M. Foucault mentioned to me in conversa- 
tion a most remarkable phenomenon which he had observed 
in the course of some researches on the voltaic are, but which, 
* Dr. Faraday showed that the red gold precipitated from solution by 
hosphorus became violet merely by the addition of chloride of sodium, 
Notices of the Meetings of the Royal Institution, June 13, 1856.) 
