PHOSPHORUS ; SULPHUR ; IRON ; ARSENIC ; SELENIUM ; TIN ; ANTIMONY ; TELLURIUM; BORON and ICE.
The methods which have been used in determining transition points are the following: (a) Dilatometric method: The solid is sealed in a bulb attached to a capillary tube; the bulb is then filled with an indifferent liquid, which just reaches to the bottom of the capillary, the whole resembling a large thermometer. When the apparatus is immersed in a bath which is slowly heated the regular rise of the liquid in the capillary undergoes a sudden alteration, corresponding with a change in the solid, at the transition tem perature, and then resumes its regular rise after a degree or so. (b) Solubility measurements: If the solubility in a given solvent is measured at a number of temperatures, the regular alteration is found to undergo a sudden change at the transition point. (c) Thermometric method: This method depends on the principles described in the case of iron. (d) Optical methods: These depend on the change of colour, crystalline form, etc. (e) Electrical methods: This may be illustrated by reference to the case of white and grey tin, for it is obvious that, in view of the electrical behaviour of two such electrodes (already described above), the current will undergo a reversal of sign at the transition point.
BIBLIOGRAPHY.—A. Smits, The Theory of Allotropy (trans. by J. S. Bibliography.—A. Smits, The Theory of Allotropy (trans. by J. S. Thomas, 1922 ; very abstruse) ; A. Findlay, The Phase Rule (1923; brief and elementary) . (A. D. M.)