16 



NINTH ORDINARY MEETING. 



Royal Institution. — 19tli Februaiy, 1855. 



JOSEPH DICKINSON, Esq., M.D., F.R.S., President, in the Chair. 



The foUowing gentlemen were balloted for, and duly elected members 

 of the Society: — William Btrom, Esq., J. S. Taylor, Esq., M.D., 

 and Alfred King, Esq. 



The President exhibited several medals recently struck by the 

 Crystal Palace Company, of very elegant design and beautiful workman- 

 ship, presented to Mr. T. C. Archer. 



The Secretary read a communication from Mr. James Boardman, 

 on the compass of modem Rome, as estimated by him by walking 

 measurement. His pace was at the rate of 3-5 miles an hour, and he 

 completed the circuit of the walls in three hours forty-five minutes, 

 showing the circumference to be 12-5 miles. 



Bearing upon the subject of the paper for the evening, a large 

 number of photographs on glass and paper, positives and negatives, 

 were exhibited ; and the following communication was made to the 

 Society, and experimentally illustrated : 



ON PHOTOGRAPHY, 



with special reference to the chemical principles involved 

 in the collodion and paper processes. 

 By J. BAKER EDWAEDS, Esq., Ph.D., F.C.S. 

 Although many ingenious processes have been devised for the pro- 

 duction of photographic impressions, none have been equally satisfac- 

 tory with those which depend for their success on the reduction of silver 

 from its salts. This mainly is the principle involved in the daguerreo- 

 type, calotype, and collodion processes, although the metal is precipi- 

 tated in modified and varied forms by each of these methods. The 

 reduction is first induced by an occult change produced in the chrys- 

 taUine or chemical structure of the silver salt by the direct action of 

 reflected light ; this change is continued and accelerated by the use of 

 chemical reducing agents, and is anrested by the application of a 

 solvent for the undecomposed silver salts. The result is a thin veil of 

 reduced metal, which may be obtained in various conditions or in 



