HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. 1135 
The expanding force is considerable. Attempts to burst an iron bottle did not 
succeed, but a thick porcelain tube was violently broken. 
An attempt to determine with accuracy the relative conductivities, for heat, of 
iodide, bromide, and chloride of silver was not successful; but the result showed that 
the bromide conducts somewhat better than the chloride, and more than twice as well 
as the iodide. 
We have seen above that iodide of silver passes at between 156°*5 C. and 142° C. 
from the transparent amorphous variety to the opaque crystalline state. Wernicke 
(Pogg. Ann., cxliii., p. 560) mentions that prisms of fused iodide when cooled to 138° C. 
exhibit a sudden alteration of colour and transparency; and he considers that at a 
high temperature “part of the iodine is separated from its combination with the silver, 
and is absorbed by the remaining substance in the liquid state, for the spectrum, like 
that of solid and liquid iodine, contains no blue or violet light. In the normal state, 
below 138° C., silver iodide gives a spectrum less bright but twice as long, and 
particularly developed in the blue-violet portion.” The supposed redistribution 
of the iodine at a high temperature was not borne out by any of our results. 
Many experiments were made in order to decide between the very contradictory 
statements in regard to the action of light upon the iodide of silver, with the general 
result that the pure iodide is quite unaffected. Even when the iodide was precipitated 
in a cell at the apex of rays of the electric light concentrated by a large lens, it did not 
darken. 
Determination of the coefficients of expansion and contraction. —Fizeau “by a 
method depending on the accuracy with which extremely minute movements can be 
appreciated, by observing the changes they produce in a system of Newton’s rings,” 
determined the coefficient of cubic contraction of iodide of silver between —10° C. 
and +70° C., and found it to be ‘00000417 for 1° C. This possession of what Fizeau 
terms a “ negative coefficient of expansion ” is the more remarkable when we remember 
that the chlorides, bromides, and iodides of potassium, sodium, and ammonium, and the 
chloride and bromide of silver, expand considerably when heated, more so, indeed, than 
the most expansible metals, such as lead, tin, and zinc. The contraction of the iodide 
of silver is, according to Fizeau, quite regular between —10° C. (14° F.) and +70° C. 
(158° F.) ; and he calculates that the contraction is equal to about ywoo of its volume 
at 0° C. for 100° C., or, again, equal to one-sixth the expansion of platinum for 100° C. 
He also found that a large hexagonal crystal exhibited a very considerable contraction 
in the direction of the axis of symmetry, while a slight expansion was produced in a 
direction normal to the axis of the crystal.* The contraction was observed in the case 
both of the crystal, a confused crystalline mass, and an amorphous mass produced by 
strongly compressing the precipitated iodide until it became a hard mass capable of 
receiving a fine polish, and possessing a specific gravity of 5‘5 6 9. Fizeau considers 
* “ Snr le propriete qui possede l’iodure d’argent de se contracter par la chaleur et de se dilater par le 
froid.” Comptes Rendus, 1867. (Tom. lxiv. p. 314.) 
MDCCCLXXXII. 7 f 
