496 



Sir J. Conroy. 



[Mar. 3, 



Silver in f Plate at A. 



carbon tetra- -l Plate at B. 

 cliloride. I. L Mean value 



Silver in r Plate at A. 



carbon tetra- ■{ Plate at B. 



Silver in air. 



Silver in water. 



Silver in carbon tetrachloride. 



Bed light. 



Yellow 



light. 



Blue light, 



P T 

 x .1. 



"P A 

 x .A. 



P.I. 



P.A. 



T> T 

 ir.l. 



Jr.A. 



/ 











o / 



74 09 



44 26 



72 05 



43 31 



65 20 



45 25 



70 43 



44 12 



71 12 



43 09 



67 33 



43 21 



72 26 



44 19 



71 38 



43 20 



66 26 



44 23 



75 11 



44 34 



72 13 



43 56 



67 10 



45 44 



70 36 



44 09 



71 10 



44 05 



67 51 



43 37 



72 53 



44 21 



71 41 



44 



67 30 



44 40 



Mean value. 











76 29 



43 51 



74 37 



43 22 



71 33 



43 



73 55 



44 02 



72 15 



44 09 



67 26 



43 26 



72 39 



44 20 



71 39 



43 40 



66 58 



44 31 



The values of the principal incidences are nearly the same as when 

 the plate had been cleaned with rouge, but the principal azimuths, in 

 addition to being considerably higher, differ but little for the three 

 kinds of light instead of increasing with the refrangibility of the 

 light. 



Films of silver chemically deposited on glass were prepared by 

 Martin's process (" Ann. de Chimie," 4th Series, vol.xv, p. 94). Glass 

 slips, similar to those used for mounting microscopic objects, were 

 well cleaned, polished with rouge, washed with water, a little nitric 

 acid poured over them, and then again washed and dried. 



A rectangular frame (about 15 centims. by 7 centims.), formed of 

 two glass rods, the ends of which were fixed into pieces of wood, was 

 placed in a shallow earthenware dish partly filled with distilled water ; 

 the surfaces of the pieces of glass which were to be silvered were wetted 

 with a mixture of equal parts of alcohol and potassium hydrate solu- 

 tion, and then the glass slips placed with their ends resting on the 

 glass rods, and with their wetted surfaces downwards. There was 

 just sufficient water in the dish for the lower surfaces of the glass 

 slips to be in contact with it, whilst the upper remained dry. 



The silvering solutions were mixed in another dish, care being taken 

 that the depth of the silvering solution was the same as that of the 

 water, and the frame with the slips resting on it, transferred from one 

 dish to the other. 



The room in which the silvering was carried on being very cold the 

 action was slow, and till at least five minutes had elapsed there was 

 hardly any deposit ; the slips were removed successively after 8, 11, 

 14, and 18 minutes, well washed with water, and placed on edge to 

 dry. The reflexion constants in air, water, and carbon tetrachloride 

 were observed, and then after the area of the silver had been measured, 

 and the slips dried and weighed, the silver was rubbed off with a damp 

 cloth, and the glass again dried and weighed, and from the loss of 

 weight and the area, the thickness of the films calculated, The 



