When, on the contrary, selenium is precipitated from some of its combinations, it assumes a cinnabar-red colour ; and in some particular cases, the colour ap proaches that of gold. When selenium is in powder, it is red ; but when pounded, it sticks together, and becomes grey.
The specific gravity of selenium is 4.32. It is a bad conductor of caloric ; a small piece of it, which is heated to near the fusing point at one end, may be held by the other end in the hand. It is a very im perfect conductor of electricity ; and, what is remark able, it has not been made electric by friction.
\Vhen selenium is heated to 212°, it becomes semi liquid, and, at a few degrees higher, it fuses. As it cools, it becomes semifluid, and then passes into the solid form. When semifluid, it may be drawn into fine threads, or beat into plates, which when thin, are transparent. These, when viewed by transmitted light, are red, by reflected light they are grey. Selenium may be made to assume the chrystalline state, though with difficulty. When slowly deposited from a solu tion of some of its salts. it puts on the dendritic form, the chrystals of which appear, by the aid of a glass, to be prisms terminated by pyramids.
When selenium is heated in close vessels, it boils, and passes off in vapour, of a colour rather darker than that of chlorine, and which condenses in the cool part of the apparatus in black drops.
When heated in the air, or in very large vessels, a red-coloured vapour is formed, which condenses in powder. When the metal is heated in contact with flame, the flame acquires at its edges a blue colour, and the selenium is volatilized, with the odour of horse raddish. The products in this case are selenic acid, and a compound of selenium and oxygen, similar to car bonic oxide. Berzelius has therefore called it selenic oxide.
Selenic oxide is procured by washing repeatedly the product formed in the above experiment. It may like wise be obtained by heating selenium in a large vessel containing oxygen, or by dissolving the compound of the metal and sulphur in nitro-muriatic acid. It is sparingly soluble in water, but communicates to that fluid its peculiar odour. It does not seem to unite with the alkalies.
Selenic Selenium combines with a larger proportion of oxygen, and forms a compound possessed of acid properties. When a stream of oxygen is passed over selenium in a state of fusion, the metal takes fire, and burns with a white flame, having a greenish tinge at the edges ; the product is selenic acid, which con denses in the cool part of the apparatus. Selenic acid is more easily procured by dissolving selenium in nitro-muriatic acid. By evaporating the solution, the undecomposed acids pass off, and selenic acid is left in the form of a white mass. When this is subjected to a higher temperature it volatilizes without becoming fluid. The temperature at which it passes into vapour seems to be below that of boiling sulphuric acid. The vapour has exactly the appearance of chlorine, and condenses in the cool part of the apparatus, in long four-sided needles.
S-..lenic acid has a sour taste, leaving a burning sen sation on the tongue. It is very soluble in cold water, and in almost any quantity of that fluid at a boiling heat. The solution on evaporation deposits acicular crystals, but by cooling slowly a saturated warm solu tion, the acid assumes a prismatic figure.
Selenic acid forms saline compounds with the alka lies, earths, and metallic oxides. It is soluble in alco hol, and yields, on distillation with this fluid, a liquid having an etherial odour ; at the same time a portion of selenium is formed, and part of the acid remains behind.
Berzelius endeavoured to ascertain the composition of this acid, by passing chlorine over the metal. in this experiment, I of selenium united with 1.79 of chlorine. According to Berzelius, 1.79 of chlorine are equivalent to 4.043 of oxygen—IGO of selenium, therefore, unite with 40.43 of oxygen, to form selenic acid.
In another experiment, the product formed by pass ing the gas over selenium was dissolved in water, and the muriatic and selenic acids generated, were precipi tated by nitrate of silver. The precipitate, after the seleniate was removed by boiling diluted nitric acid, weighed 7.2285 = 1 38 of muriatic acid = 40.274 of oxygen. Accoiding to this experiment, 100 of selenium combine with 40.274 oxygen.