[ 1169 J 
XXVII. On the Specific Heat and Heat of Transformation of the Iodide of Silver , 
Agl, and of the Alloys , or Compounds, Cu 2 I 2 .AgI; Cu 2 I 2 .2AgI; Cu 2 I 2 .3AgI; 
Cu 2 I 2 .4AgI; Cu 2 I 2 .12AgI; PbI 2 .AgI. 
By Professor Maneredo Bellati and Dr. B. Bomanese, of the 
University of Padua. 
Communicated by Professor A. W. Williamson, Foreign Secretary R.S. 
Received Jane 7,—Read June 15, 1882. 
In a series of papers printed in the Proceedings of the Boyal Society for the years 
1877, 1879, 1881, Mr. G. F. Bod well has investigated the expansion and contraction 
by heat of the silver iodide, Agl; of certain chlorobromiodides of silver; and of certain 
alloys of silver iodide, with lead iodide, and copper iodide. Analogous researches were 
made by us on the substances HgI 2 .2AgI; HgI 2 ,3AgI; HgI 2 .Cu 2 I 2 ;* which, when 
heated to a certain temperature, change their colour. HgI 2 .2AgI and HgT 2 .3AgI, at 
about 50° C., change from canary-yellow to red, and HgI 3 .Cu 2 I 2 , at about 70° C., 
changes from red to a chocolate colour. For temperatures below and beyond the 
colour-change the co-efficient of expansion and the specific heat are regular, but during 
a range of a certain number of degrees of heat in which colour-change and corresponding 
modification of structure take place, the substances undergo a very notable expansion 
and absorb a great quantity of heat. 
It seemed to Mr. Bodwell, and to us, that it would be interesting to make a 
calorimetric study of those substances whose expansion and contraction he had deter¬ 
mined, and Mr. Bodwell having kindly furnished us with the specimens which he 
employed in his researches, we have briefly described in this paper the method and 
results of our calorimetric determinations. 
During the operation of heating, the substance was contained in a vertical double 
cylinder of brass, 3 centims. in diameter and about 12 centims. long (fig. 1, p. 1176). 
The substance to be examined, made into small rods, was arranged round the reservoir 
of a thermometer, whose stem passed through a cork which closed the upper end of the 
brass cylinder. At the lower end this cylinder was closed by two small double doors 
which could be opened by a spring, and thus allowed the substance to fall out. The 
* M. Bellati and R. Romanese. Atti del R. Tstit. Venefco (1880), ser. v., vol, vi.; Nuovo Cimentg 
(1880), ser. iii., vol. viii. 
7 K 2 
