192 YEAR-BOOK OF FACTS. 



explanation of the chemical changes involved, I beg to submit the 

 following results of a series of experiments, made at intervals of 

 leisure during the summers of 1857, 1858, and 1859, which would 

 appear strongly to favour the first-mentioned hypothesis. 



****** 



Passing in review the results obtained in the foregoing experi- 

 ments, it will probably be considered that the weight of evidence 

 tends to show that the metal is the ordinary product of the chemical 

 action of light upon chloride of silver ; .and that the principal diffi- 

 culty which has stood in the way of accepting this conclusion bus 

 in a great measure to be accounted for by the often varying shades 

 of colour presented by the reduced metal, and more especially the 

 transition observed at the moment of removing the unaltered por- 

 tion of material by the application of the fixing agent. If in these 

 several stages the change in physical condition be considered in its 

 proper connexion, and due allowance be made for the very impor- 

 tant influence known to be exercised over the light-reflecting 

 capacity of these minutely divided particles by very slight modifica- 

 tions in their state of aggregation (quite irrespective of Chan 

 chemical constitution), there will then be no longer any difficulty in 

 referring these results, with others of the same class (c. </., the BOVB* 

 ral 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 substantiated 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, ll.ml- 

 wich, Llewellen, and Maskelyne, in their joint report upon this 

 subject recently presented to the meeting of the British Associa- 

 tion. 



Propositions. — 1st. That chloride of silver, when decomposed by 

 light, is separata '1 into its elements. 



2nd. That this change does not usually extend to the whole bulk 

 of the material operated upon, on account of the opaoity of the 

 darkened product mechanically protecting a certain portion of un- 

 altered chloride of silver from the action of the light. 



8rd. That the degree and rapidity of redaction is influenced by 

 the Btate of division of tin- particles, ami by the presence of 

 capable "i absorbing the chlorine when liberated from i*s combina- 

 tion with silver. 



PHOTOOBAPH1 a cknu'ky | 

 In the London Review, I 1 ''-. L5, I860, attention is called to a 



14 foreshadowing'' of tin 1 Photographic Art, in a I k entitled 



Oiphantit & Babylone, published in 17''* 1 ; ami in which the author, 

 Tiphaigne de la Roche, relates the wonders which were revealed to 



