Physical Properties of Cadmium
|Stromeyer noted the resemblance of his new metal to tin in its colour, brightness, softness, ductility, and in its " cry " when twisted. Cadmium is silver-white tinged with blue, and its bright lustre soon dulls in air. |
Cadmium is said to occur in three allotropic modifications. Silver-white crystals of cadmium, in octahedra and other forms, sublimed when the metal was distilled in hydrogen, and in flat needles or six-sided tablets when it was distilled in vacuo. The metal apparently crystallises isomorphically with zinc in the hexagonal system, and holohedrally.
The cast metal is crystalline, and when cadmium is bent it " cries " like tin.
The density of distilled cadmium is 8.64819 at 20°/4° C., which becomes 8.64766 after compression, but it varies with the history of the metal and usually diminishes on " working."
When cadmium is drawn into wire its density alters from 8.6434-8.6397.
According to Topler, 1 grm. of cadmium expands to 0.0064 c.c. on fusion, and the density of molten cadmium has been represented by the expression
8.02 – 0.00110 (t - 320),
where t is the temperature.
The vapour density of cadmium corresponds to a monatomic molecule. The metal also depresses the vapour pressure of mercury, in which it is dissolved, as if it were monatomic.
The melting-point of cadmium is 320.9° C. It rises 0.006288° C. for every pressure rise of 1 atmosphere. The latent heat of fusion per gram-atom is 1.570 Cal.
Berthelot found that cadmium boiled at 778° C. Previous determinations had varied from 746°-860° C. Egerton accepted Berthelot's figure, though Heycock and Lamplough had found 765.9° C.
In vacuo cadmium slowly volatilises at 160° C. and boils at 450° C. The boiling-point alters by 0.12° C. per each mm. change from normal pressure.
According to Fogler and Rodebush,16 the latent heat of vaporisation of cadmium at 321.1° C. is 25.350 ± 0.100 Cal., and its vapour pressure varies from 0.10 mm. At 321.1° C. to 760 mm. at 766° C.
The specific heat of cadmium varies from 0.04907 at -164.7° C. to 0.05714 at 97.8° C. The specific heat of distilled cadmium is 0.0559, which becomes 0.0560 after compression.
Pure cast cadmium, according to Jager and Diesselhorst, has a thermal conductivity of 0.222 at 18° C. and 0.216 at 100° C. According to Lees, pure redistilled cast cadmium varies from 0.240 at -170° C. to 0.217 at 18° C.
The electrical conductivity of cadmium in reciprocal ohms per centimetre cube varies from 15.5×104 at -170° C. to 13.9×104 at 18° C.
The coefficient of linear expansion was given by Schaefer as 0.00003060. Unit volume at 0° C., according to Matthiessen, becomes l+0.00008078t+000000014t2 vols, at any temperature t between 0° C. and 100° C.
The arc spectrum of cadmium contains the following more important lines, expressed in 10-8 cm. units: 3261, 3404, 3406, 3466, 3611, 3982, 4413, 4678, 4799, 908, 5085, 822, 5155, 5338, 5379, 6438, 470.
|Cadmium is dispersed as a deep brown colloidal solution when an arc is obtained between cadmium electrodes immersed in pure, air-free water. The colour changes to blue-green and electrolytes coagulate the solution. Exclusion of air and thickening with gelatine preserve the solution for some time. |
Svedberg, by an electrical method, obtained a stable dispersion of cadmium in isobutyl alcohol that was brown by transmitted light and greyish black by reflected.