144 



On the Coefficients of Expansion, fyc. [Dec. 8, 



601° C, the highest melting point of any one of the alloys is 514° C, 

 and the lowest 493° C. 



3. Some of the alloys possess three points of similar density, and 

 some two, at different temperatures. They are resinous in fracture 

 and transparent in thin layers. When pulverised they furnish 

 brilliantly yellow powders, unaffected by light. 



4. When heated in a current of carbonic anhydride they volatilise 

 very slowly. Heated in dry oxygen iodine is freely evolved, and oxide 

 of copper appears on the surface of the mass. When heated in dry 

 hydrogen, hydriodic acid is produced, and the metal is reduced. 



5. The coefficients of expansion of the alloys below the point at 

 which contraction on heating commences, was found to decrease as the 

 percentage of iodide of silver was augmented. Thus — 



Percentage of Agl. Coefficient of expansion. 



38-2232 -00004998 



553066 -00003750 



64-9884 -00002307 



71-2225 • -00001998 



88-1304 -00000636 



The same fact was observed in the case of the chlorobromiodides of 

 silver. A curve table shows results. 



6. While the iodide of silver commences its considerable contrac- 

 tion at 142° C, the five chlorobromiodides of silver, the percentage 

 of iodide of silver in which varies from 26'1692 to 73'9285, and the 

 lead- silver iodide alloy, the percentage of iodide of silver in which 

 amounts to 33" 794, all commence their contraction at 124° C, that is, 

 18° C. lower, although the coefficients of expansion of the associated 

 bodies necessarily differ. Thus it would appear that 124° C. is the 

 temperature at which iodide of silver commences its passage from the 

 crystalline into the amorphous condition, when freed from the attrac- 

 tion of its own molecules, provided no other attraction or influence 

 supervenes ; while the attraction exerted when it exists unalloyed with 

 any other substance, and when its molecules are hence much nearer to 

 each other, raises the point of commencement of the change to 142° C. 



7. The probable cause of this is discussed. 



8. When the same result was looked for in the case of the copper- 

 silver iodide alloys it was not found. In fact, the presence of the 

 iodide of copper, instead of promoting the assimilation of molecular 

 motion, and lowering the point at which the change from the crystal- 

 line into the plastic condition commences, was found to considerably 

 raise it, although the coefficient of expansion of the iodide of copper is 

 lower than that of either chloride or bromide of silver, or of the iodide 

 of lead, which enter into the composition of the other alloys. 



