|Chromium was not for some considerable period obtained in a pure state, being always contaminated with carbon, silicon, aluminium, iron, etc., incidental to the method of preparation? and investigation of Physical Properties of Chromium was difficult. That obtained by Vigouroux contained 0.36 to 0.4 per cent, silicon and 0.74 to 0.85 per cent, aluminium and iron, though it was free from carbon. Moreover, some processes furnished the impure metal in an amorphous or porous condition, while in others the crystalline form was produced. Thus it is evident that a certain discord among the observed data is to be anticipated. The appearance and form of pure chromium have been variously described; for example, as grey rhombohedra; as tetragonal pyramids; as a microscopically crystalline grey powder; as tin-white rhombohedra; as fusible crystals. A recent X-ray examination of the crystal structure of a specimen of chromium has revealed the presence of two allotropes of the metal. Moissan has prepared a pyrophoric form by distilling the amalgam at 300° C. in vacuo. |
The hardness of metallic chromium measured on Rydberg's system is 9.0, though pure chromium prepared by Moissan did not scratch glass. The density of chromium at 16° C. is 6.72, while for the pure metal (previously fused in the electric oven) at 20° C. the value 6.92 was obtained. Jassonneix obtained the value 71 at 17° C. for a specimen obtained from the boride. The mean specific heat of metallic chromium between 0° C. and 98.24° C. is 0.1216; between 0° C. and 100° C., 0.1208. The specific heats at temperatures ranging from 0° C. to 500° C. of chromium, containing 1.3 per cent. Fe and 0.09 per cent. Si, have been determined as follows:
|Temperature, °C.||Specific Heat.|
Electrolytically deposited chromium occludes 250 times its own volume of hydrogen. At ordinary temperatures chromium does not exhibit magnetic properties, but does so at -15° C. to -20° C. The electrical conductivity of powdered chromium is approximately 38.5 mhos per cm3.
The melting-point of the metal has been variously recorded as 1489° C., 1513° C., 1553° C., 1547° C., and 1550° C. Like certain other Physical Properties of Chromium, the melting-point is profoundly affected by the presence of impurities. Thus a sample of chromium made by the alumino-thermic process and containing 1 per cent, of impurity was found to melt at 1515.5° C. The boiling-point of chromium is 2200° C. It can be distilled in the electric furnace, when a crystalline variety is obtained having the same chemical properties as the finely powdered metal.