TIN FROM CASSITERITE. 21
This method, on account of the relatively high cost oi potassium cyanide,
is used only in the laboratory or in the technical reduction of very valuable
metals from their oxides.Heat a mixture of 20 g. of very finely-powdered cassiterite and
20 g. of potassium cyanide in a porcelain crucible, which should be
about three-quarters filled thereby, for half an hour over the
blast lamp. A clay mantle may be placed around the crucible
to lessen the amount of heat lost by radiation. After cooling,
wash the regulus of metallic tin (12 to 15 g.) with water. Express
the yield in per cent of the weight of mineral taken. Test the
tin qualitatively for iron, copper, and lead, and determine its
specific gravity. The specific gravity of pure tin (in the white
modification) is 7.287 at 15°.Melting-point Determination.
The difference in the energy content of a substance in different states of
aggregation is a far more distinctive property than the mere outward char-
acteristics, — solid, liquid, and gaseous. Such energy differences can be meas-
ured in the heat of fusion or of vaporization.
The melting-point of a substance can, therefore, be determined very ac-
curately if it is uniformly heated and the changes of temperature, as
shown by an inserted thermometer, are observed. As soon as the melting-
point is reached, the mercury remains stationary until all of the metal is
melted even although the outer temperature is higher; the heat supplied to the
substance is all utilized in causing its fusion before any is available for produc-
ing a further rise of temperature. If the outer temperature is only a few
degrees higher than the melting-point, the temperature of the substance
remains constant for a considerable time, so that the thermometer may be
read accurately. On cooling a fused substance, the thermometer likewise
remains constant for some time at the solidification point, even when the outer
temperature is lower. With pure substances the melting-point and the
solidification-point are identical.
Cut the tin into a few small pieces and place it in a test tube
of 1 cm. diameter which dips 4 cm. into a small beaker filled with
sulphuric acid. Cover the beaker with a disk of asbestos board
provided with a hole into which the test tube fits. Heat the
sulphuric acid until the tin just melts; then heat a thermometer
cautiously over a flame to about 200° and dip it into the molten
metal. Lower the flame under the beaker so that the temperature
of the bath falls slowly, and, while stirring, observe the changes
of the thermometer reading. At the solidification point the tern-