i 162 
MR. G. F. RODWELL ON" THE EFFECTS OF HEAT OH CERTAIN 
admitting the supposition that alloys are solidified solutions of one body in another 
(12), this lessening of cohesion is not difficult to explain. For it is well known that 
particles of matter of similar composition attract each other ( cohesion) more readily, 
and with greater force, than particles of different composition {adhesion), and hence 
the homogeneous diffusion of one substance in another must make the molecular 
attraction less than that of either body by itself. Hence, molecular motion is more 
readily assimilated, and sooner produces any given effect, such as the fusion of the 
mass. 
14. It was thought that the same explanation would apply to the curious fact that 
the five chlorobromiodides of silver, and the lead-silver iodide alloy, all commenced their 
period of contraction on heating at a lower temperature than the iodide of silver, to 
which substance alone can they owe their contraction on heating, but the results 
obtained with the copper-silver iodide alloys appear to discredit this. 
15. For it is most noteworthy in this connexion to observe that while the iodide of 
silver commences to contract considerably on heating, that is to pass from its crystalline 
into its amorphous plastic condition, 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° 0., that is 18° C. lower, although the coefficients 
of expansion of the associated bodies necessarily differ. It would thus 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 attraction 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 
change 18° C. higher, viz.: to 142° C. 
16. The same result was looked for in the case of the copper-silver iodide alloys, but 
it was not found. The temperature at which contraction commences was in all cases 
raised, instead of being lowered, so that the presence of the iodide of copper hinders 
the assimilation of molecular motion by the molecules, although the coefficient of 
expansion of the iodide of copper is lower than that of either the chloride or bromide 
of silver, or of the iodide of lead which enter into the composition of the other alloys. 
Percentage of iodide of copper 
in the silver-copper iodide alloys. 
61*7767 
44-6934 
35-0116 
28-7775 
11-8696 
Temperature at which 
expansion on heating finishes. 
256° C. 
221° C. 
194° C. 
180° C. 
124° 0. 
