HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. 1133 
then filled with the substance under examination at its melting-point; the substance 
was allowed to solidify, and the whole was weighed. The cavity due to the con¬ 
traction of the mass in solidifying was then filled with mercury to the level of the 
mouth of the conical tube, and the mercury was weighed. Then, knowing the capacity 
of the tube, the specific gravity of mercury, and of the substance under examination, the 
temperature of the fusing-point, and the coefficient of expansion of the platinum cone, 
we have all the data requisite for the determination. A glass tube was sometimes 
employed. The real difficulty is the determination of temperatures above the boiling- 
point of mercury; and until a trustworthy method of general application has been 
devised, such determinations must be regarded as approximations. The expressions 
“below a red heat,” “a dull red heat,” &c., are still common in text-books and in 
memoirs; but the very definitions of what is meant by a “'dull red heat” vary, 
as also do the temperatures assigned to it by different waiters. The method of 
Mr. Carnelley (Jour. Chem. Soc., 1876) appears to be the most accurate yet devised, 
3. Iodide of Silver, Agl. 
Experiments were made in order to determine certain general physical properties of 
iodide of silver before placing it in the expansion apparatus. 
It was prepared :— 
(a) By precipitation according to the equation 
AgN0 3 +KI=AgI+KN0 3 . 
A dilute solution of pure iodide of potassium was added to a dilute solution of 
nitrate of silver. The precipitated iodide was thoroughly washed with boiling dis¬ 
tilled water, slowly dried, fused rapidly in a porcelain crucible, and cast into cylindrical 
rods in warm thin tubes of well-annealed glass. 
(/3) By dissolving pure silver (for which my thanks are due to Professor Chandler 
Roberts) in strong hydriodic acid, evaporating to dryness, and fusing. 
(y) By exposing silver leaf to the spontaneous evaporation of iodine. 
The iodide prepared by precipitation was commonly used for all the purposes 
described in the following pages, with the exception of the experiments relating to the 
effect of light upon the iodide.. 
General physical properties .—The molten iodide which forms a mobile bromine-red 
liquid, solidifies at 527° C. to a perfectly transparent, very flexible, claret-coloured solid; 
as it cools it becomes lighter in colour, and the colour of amber, and as 163° C. is 
approached it becomes pale yellow. All this time it has been slowly contracting on 
cooling. At 163° C. the contraction ceases, and the mass neither expands nor contracts 
till the temperature has fallen to 156°‘5 C., when expansion commences, slowly at first, 
but becoming very rapid between 151°*3 C. and 148° C. The body simultaneously 
changes from a plastic transparent, amorphous state, to that of a crystalline, opaque. 
