HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. 1127 
On cooling, the mercury rose in the tube, until the contraction of the mercury exceeded 
the expansion of the iodide. 
Thus it was obvious that some method of examination must be adopted, which 
should be equally applicable to the indication, and measurement, of either expansion or 
contraction. 
Three experimental methods were tried. The last (y) was finally adopted. 
a. Method of the specific gravity flash —Professor Guthrie suggested to me that a 
convenient mode of determining the contraction of the iodide might be to fill a specific 
gravity flask with mercury, and to determine the amounts of mercury driven through 
the capillary tube of the stopper for every ten or twenty degrees increase of tempera¬ 
ture ; then to place in the flask a known weight of fused iodide of silver, together 
with a known weight of mercury, and to repeat the determinations for similar ranges. 
440 grms. of mercury were thus heated, and weighings were made at intervals of 10° C.; 
afterwards 38*3680 grms. of iodide of silver were introduced, and the heating maintained 
through similar ranges; but it was found that as the temperature approached 7 0° C. 
the iodide of silver was slightly decomposed by the hot mercury, green protoxide of 
mercury, Hgl, being formed. Thus this method had to be abandoned. 
j3. Method of the microscope .—It was thought that rods of the iodide might be 
placed in a vertical bath of paraffine, and examined in the field of a microscope fitted 
with a micrometer eyepiece, during heating through various ranges of temperatures. 
The following apparatus was constructed for this purpose:—Two blocks of white 
marble (A, A, fig. A), 8 inches high by 4 inches by 3 inches, were placed 15 inches 
Fig. A. 
apart. Between them the paraffine bath B of copper was supported on massive iron 
bars C, C, terminated by levelling screws which rested on the marble blocks. The 
length of the bend in the iron bars, and the height of the brass levelling screws 
between the marble and the iron bars were so arranged that the upward expansion of 
the one compensated the downward expansion of the other. The bath contained 
ceresine capable of being heated to 340° C. A frame fitted into the bath containing 
MDCCCLXXXII. 7 E 
