The latter variety of selenium, like the crystalline form of sulphur, dissolves in bisulphide of carbon, but much less readily. Selenium boils below a red heat and becomes converted into a deep yellow vapour, which, when heated, is subject to the same anomalous expansion as sulphur vapour. It is not so combustible as sulphur, which it still further resembles by burning with a blue flame when ignited in the air. During combustion it gives off a peculiar and characteristic smell, resembling that of putrid horse-radish. Heated with strong sulphuric acid, selenium forms a green solution. If this solution is poured into water, the selenium separates and is thrown down. Selenium is without taste or smell, is insoluble in water, and in its normal state is a non-conductor of heat and electricity. Selenium may be extracted from the Fahlun residue by the following process:—It should be first boiled with sulphuric acid, diluted
with an equal volume of water, and nitric acid should then be added in small quantities until the oxidation of the selenium is accomplished, which may be known when red fumes cease to be cooled. The solution which contains selenious (SeO2) and selenic (SeO3) acid is then to be largely diluted with water, filtered, the filtrate mixed with about one fourth of its bulk of hydrochloric acid, and then concentrated a little by evaporation, the result of which is that the hydrochloric acid reduces the selenic to selenious acid. A current of sulphurous acid being then passed through the solution, the selenium is precipitated in red flakes, which form into a dense black mass when the liquid is gently heated. The following equation illustrates the reaction:—
H2O,SeO2 + H2O + 2SO2 = 2(H2SO3) + Se.
Like sulphur, selenium combines with oxygen and forms an anhydride corresponding to sulphurous anhydride. Selenious anhydride (SeO2) may be obtained by burning selenium in a current of oxygen; it is, however, more easily prepared by boiling selenium with nitric acid or with aqua regia, the excess of acid being expelled by heat, the selenious anhydride is left as a white mass. When this is dissolved in water it yields a crystalline hydrate of selenious acid (H2SeO3). The salts formed by selenious acid (selenites), with the exception of those of the alkali metals, are mostly insoluble in water. They are easily known by the peculiar odour of selenium which they give off when heated on charcoal in the reducing flame of the blowpipe; solutions of the selenites give a reddish-brown precipitate when treated with sulphurous acid.
Seleniuretted Hydrogen (H2Se). This may be obtained in a precisely similar manner, namely, by acting on selenide of iron or potassium with diluted sulphuric or hydrochloric acid. Seleniuretted hydrogen is soluble in water, and precipitates many metals from their salts as selenides. The solution is feebly acid, and, like its analogue solution of sulphuretted hydrogen, if exposed to the air, absorbs oxygen and deposits selenium. The selenides of the alkali metals are soluble in water. The selenides of cerium, zinc, and manganese are flesh-coloured; most of the others are black. This gas is inflammable like sulphuretted hydrogen; it has, however, a still more offensive smell than this latter gas, Berzelius lost his sense of smell for several hours by the application to his nose of a bubble of seleniuretted hydrogen not larger than a pea. There are two chlorides of selenium—a dichloride (Se2Cl2), a volatile liquid of a brown colour, and a tetra-chloride (SeCl4), which occurs as a white crystalline solid. Selenium unites with sulphur, forming a bisulphide (SeS2) and a tersulphide (SeS3). A very curious physical property of selenium when exposed to the action of light was first noticed in 1873 by Mr May, assistant chemist
at the Telegraph Station at Valentia, in Ireland, who observed that a stick of crystallised selenium which had been used for some time in telegraphy, where high electrical resistance was required, offered a considerably diminished resistance to the current when exposed to the light than when kept in the dark. Mr May’s discovery, which was at first received with some amount of incredulity, has since been amply corroborated by the observations and researches of many physicists, amongst them by Professor Werner Siemens, the result of whose experiments on this interesting subject we quote from a lecture delivered at the Royal Institution by his brother, Dr William Siemens, in February, 1876. After describing the method by which his brother arranged the selenium, so that, when inserted in the galvanic current of a single Daniell’s cell, the surface action produced by the light upon it attained a maximum effect, and thereby did away with the necessity of employing a large galvanic battery, and at the same time allowed an ordinary galvanometer to be used instead of a delicate one, as hitherto employed, Dr Siemens proceeded to illustrate the action of light upon the element by experiment. “I here hold,” he said, “an element so prepared of amorphous selenium, which I place in a dark box, and insert in a galvanic circuit comprising a Daniell’s cell and a delicate galvanometer, the face of which will be thrown upon the screen through a mirror by means of the electric light.
“In closing the circuit it will be seen that no deflection of the needle ensues. We will now admit light upon the selenium disc and close the circuit, when again no deflection will be observed, showing that the selenium in its present condition is a non-conductor both in the dark and under the influence of light. I will now submit a similar disc of selenium which has been kept in boiling water for an hour and gradually cooled to the same tests as before. In closing the circuit while the plate is in the dark a certain deflection of the galvanometer will be discernible, but I will now open the lid of the box so as to admit light upon the disc, when on again closing the circuit a slight deflection of the galvanometer needle will be observed. In closing the box against the light this deflection will subside, but will again be visible the moment the light is readmitted to the box. Here we have, then, the extraordinary effect of light upon selenium clearly illustrated.
“I will now insert into the same circuit another selenium plate which has been heated up to 210° C, and, after having been kept at that temperature for several hours, has been gradually cooled; it will be observed that this plate is affected to a greater extent than the former by the action of light, and other conditions, to which I shall presently allude, prove the selenium heated to a higher temperature to be in other respects dissimilar to the other
two modifications of the same. These differences will be best revealed in describing my brother’s experiment. He placed one of his amorphous preparations of selenium in an air-bath heated above the melting point of selenium (to 260° C.), while the connecting wires were inserted in a galvanic circuit consisting of only one Daniell’s element and a delicate reflecting galvanometer, and every five minutes the temperature and conductivity of the selenium were noted. Up to the temperature of 80° C. no current passed; from this point onward the conductivity of the material rapidly increased until it obtained its maximum at the temperature of 210° C., being nearly its melting point, after which an equally rapid diminution of conductivity commenced, reaching a minimum at a temperature of about 240° C., when the conductivity was only such as could be detected by a most delicate galvanometer. In continuing to increase the temperature of the fluid selenium very gradually but steadily, its conductivity increased again.
The interpretation of these experiments is as follows: Amorphous selenium retains a very large amount of specific heat, which renders it a non-conductor of electricity: when heated to 80° this amorphous solid mass begins to change its amorphous condition for the crystalline form, in which form it possesses a greatly reduced amount of specific heat, giving rise to the increase of temperature beyond that of surrounding objects when the change of condition is once set in. If care is taken to limit the rise of temperature of the selenium to 100° C., and if it is very gradually cooled after being maintained for an hour or two at that temperature, a mass is obtained which conducts electricity to some extent, and which shows increased conductivity under the influence of light. But in examining the conductivity of selenium so prepared at various temperatures below 80°, and without accession of light, it was found that its conductivity increases with rise of temperature, in which respect it resembles carbon, sulphide of metals, and generally electrolytes. This my brother terms his first modification of selenium.