34 CHANGES OF CONDITION.
(b) The temperature is found at which the difference of potential between
each of the two modifications and some common electrolyte is the same; in
other words, the electromotive force of an element constructed from the two
modifications and a common electrolyte is equal to zero.- Allotropy of Silver Sulphide.
Silver sulphide has a transition temperature of above 170°, which can be
determined both from the electrical resistance and from the cooling curve.
The phenomenon of supercooling not being very pronounced with this sub-
stance, a point of inflection is found on the cooling as well as on the heating
curve; on the former it lies naturally at a lower temperature than on the
latter.
In order to prepare silver sulphide, treat a hot solution of about
20 g. of silver nitrate in 300 to 400 c.c. of water with hydro-
gen sulphide, and wash the precipitate by decantation. After250
240°
S30°
220°
210°
200°
190°
180°
170°
160°
150°'•\100 200 300 400 500
Seconds220°
210°
200°
190°
180°
170°
160°
160°
140°
130°/ /100 200 800 400 600 000
Seconds
FIG. 10. Cooling Curve. FIG. 11. Heating Curve.removing the liquid by suction, dry the preparation in the hot
closet. Silver sulphide thus formed is not absolutely free from
uncombined sulphur.
Place about 8 g. of the silver sulphide in a test tube and insert a
thermometer. Heat the test tube to about 290° in an air bath,
consisting of a porcelain crucible covered with a piece of asbestos
board. Some sulphur sublimes, but this does not interfere with
the experiment. Then transfer the test tube quickly to a small
beaker containing sulphuric acid or paraffin at 145°, which tem-
perature must be kept constant to within a few degrees throughout