SELEXKTHYL. 



SELF-REOISTKKIXO INSTRUM KXTs. 



li as i Hi is during the night (Horn.. ' Hymn.,' xxxii. 7.) Her chariot is 

 usually represented in ancient works of art drawn by two horses, 

 whereas that of Apollo was drawn by four. She is only distinguished 

 from Artemis by being more fully draped. She is frequently re- 

 presented in bas-reliefs on sarcophagi, hovering in the air over the 

 slseping Endyiuion. In course at time the attributes of Selene were 

 given to Artemis, and the latter was represented s* the goddess of the 

 moon ; but these deities were originally distinct [ ARTIUHS.] 

 SELEXETHYL. [SMJWit'jr] 



SELEXJC ACID. 



SELEXIOUS ACID. [Ssunrrosl.1 



SELEXITE. U/AUIIM. Hnlplitirie Arid and Lime.] 



SELENIUM (So) a nonuietallic. solid, elementary body, discovered 

 in 1318 by Benelius in the iron pyrites of Fahlun ; the sulphur pro- 

 cured from which was used at Qripsholm in the manufacture of sul- 

 phuric add. In the latter a deposit was formed of a red colour, which. 

 on account of the peculiar odour that it emitted, was supposed, though 

 erroneously, to contain tellurium, the origin of which name suggested 

 that of trlfitium, from 3<A*ni. the moon, for the new substance. The 

 discoverer considered it a metal, but it is now classed with the non- 

 mrUllic elements. 



Selenium has been found hitherto only in small quantity ; it occurs 

 in the pyrites of Anglesey, and probably in that of many other places : 

 sulphide of selenium has been detected among the volcanic product* 

 of the Lipari Isles ; and in the Han it has been met with, combined 

 with lead, silver, and some other metals. 



Magnus extracts selenium from the native sulphide by treating 

 it with binoxide of manganese, by the oxygen of which the sulphur 

 U converted into sulphurous acid, which escape* in the gaseous 

 form, while the selenium either sublimes in its pure state or as 

 elenious acid. 



Selenium is a brown somewhat transcluceut solid when in mass. It 

 is inodorous and insipid, moderately hard, may be readily scratched 

 with a knife, is brittle as glass, and easily reduced to powder. Its 

 fracture U conchoidaL Its sp. gr. varies from 4-30 to 4-8, on account 

 of the cavities which it frequently contains. It is a bad conductor of 

 heat, and a non-conductor of electricity. It softens at 212, and may 

 be drawn out into fine threads, which are transparent, and of a red 

 colour by transmitted light. When heated rather higher, it becomes 

 fluid, and boils at 650, emitting a vapour, which is inodorous, and of 

 a deep yellow colour ; this in close vessels condenses in dork globules 

 of a metallic lustre, or of a cinnabar-red colour, according as the space 

 in which it collects is small or large. Water does not dissolve selenium ; 

 it is however soluble in the fat oils and melted wax, but not in the 

 volatile oils. The equivalent of selenium is 3975. 



Oxygen and Selenium combine in three different proportions, form- 

 ing oxide of selenium, selenious acid, and selenic acid. 



Oxide of Selenium (SeO) may be formed by heating the selenium in 

 a limited quantity of atmospheric air, and by washing the product to 

 separate the selenious acid formed with it It emits a very strong 

 smell resembling that of decayed horse-radish, so that l-50th of a 

 grain of selenium is sufficient when burnt to scent a room of consider- 

 able size, and this is a characteristic property of selenium. Oxide 

 of selenium is gaseous, colourless, very slightly soluble in water, and 

 quite devoid of acid properties. 



vfeaiow acid (SeO,) may be prepared by passing a current of oxygen 

 gas over heated selenium ; but it is more conveniently obtained by 

 digesting selenium in nitric acid or nitro-hydrochloric acid till dis- 

 solved, and then evaporating the solution to dryness. 



This acid is colourless, and when strongly heated sublimes, and con- 

 denses unchanged in the form of acicular crystals, which possess 

 distinctly marked acid properties. It attracts moisture from the air 

 and is consequently very soluble in water ; a hot saturated solution 

 yields crystals on cooling ; it is also soluble in alcohol, and has when 

 heated an acrid odour. It forms salts with bases, which are called 

 Sdenita. 



Many substances which have strong affinity for oxygen decompose 

 elenious acid ; this is the case with sulphurous acid and phosphorous 

 acid, and when the former is added to a solution of selenious acid, a red 

 powder is precipitated, which U pure selenium, and sulphuric acid is 

 at the same time formed. An alkaline sulphate produces a similar 

 effect; hydrosulphuric acid U also decomposed by and decomposes 

 selenious acid, and a yellow compound is formed and precipitated, 

 which u sulphide of selenium. 



Stiexic Acid (HO. SeO,) may be prepared by fusing selenium, a sele- 

 nide, or selenite, with nitrate of soda. The seleniate of soda obtained 

 is to be decomposed by nitrate of lead, and the insoluble seleniate of 

 lead precipitated is to be decomposed by a current of hydrosulphuric 

 acid, which throws down the lead as a sulphide. The selenic acid 

 remains in solution, with some excess of hydrosulphuric acid, which is 

 to be expelled by ebullition. 



Selenic acid u liquid, colourless, inodorous, of an oleaginous consist- 

 ence, and very acid ; it attracts moisture from the air, and always 

 retains about 12'4 per cent, of water, which cannot be expelled with- 

 out decomposing the scid. It may be heated to 536 without . i 

 position, but at 54 it is rapidly converted into oxygen and selenious 

 acid. When concentrated at a temperature of 329?, its up. gr. U 2-524, 

 sod it gradually increases up to the temperature of 64.V, when it 



becomes 8*625. When mixed with water, much heat is evolved. It* 

 salts are called Scleaiatet. Zinc and iron are dissolved, by this acid 

 with the evolution of hydrogen gas, and copper with the formation 

 of selenious acid. 



Sdatiale of toda (NaO, SeO, + 10 aq.) occurs in large crystals like 

 those of Glauber's salt Seleniate* of lime (CaO, SeO, + 2aq.) and of 

 xirtel ( NiO, SeO, -f 6aq. ) resembles the corresponding sulphates. 

 Meniale of Cadmium (CdO,SeO s + 2aq) differs both in composition 

 and crystalline form from the sulphate of cadmium, the crystals also 

 lose an equivalent of water on being heated to 212'' Fahr. 



Hydrogen and Sttexium (HSc) unite to form hydroselonic acid or 

 seleniuretted hydrogen. It is easily obtained by the action of diluted 

 sulphuric acid upon selenide of potassium, or of iron. This gas ia 

 colourless, has a disagreeable odour, and so powerfully irritates the 

 membrane lining the nose as to excite symptoms of catarrh, and 

 destroy the sense of smelling for some hours. Water readily dissolves * 

 this gas : the solution is at first colourless, but after a time it acquires 

 a roddish hue ; it gives a brown stain to the skin, and reddens litmus 

 paper. When exposed to the atmosphere it is decomposed, its hydro- 

 gen being absorbed by oxygen, and the selenium U deposited. It 

 decomposes the solutions of many metallic salts, selenides of the 

 metals being precipitated. 



Chlorine and Selenium combine when the gas is passed over the 

 selenium ; heat is evolved, and a brown chloride is obtained, which 

 is liquid ; it is heavier than water, not very volatile, and by the a 

 of water is eventually decomposed, and resolved into sclenious and 

 hydrochloric acids and selenium. 



This difhloride (80,01) may be converted into a bicliloritle by exposure 

 to excess of chlorine. It is a white solid compound which is volati- 

 lised when heated, and condenses in small crystals. It is decom- 

 posed by water. 



Sulphur and Selenium (S,Se). A definite compound of these is 

 formed by passing hydrosulphuric acid gas into a solution of hydro- 

 selenic acid ; the fluid assumes a yellow colour, becomes turbid, and a 

 precipitate is formed, though but slowly, unless a little hydrochloric 

 acid be added. When exposed to heat this dUulphide of selenium 

 becomes red and viscid, and at high temperatures may bu distilled 

 without decomposing. 



Photplwria and Selenium combine when the selenium U dropped 

 into melted phosphorus ; the product is a red substance, but it does 

 not appear that any definite compound of these bodies U formed. 



Selcxethyl or tdmium-tthyl. [ETHYL, selenide of et/ti/l.] 



Selenium forms sdcnidc* with most of the metals, and the selenide 

 of lead is one of the most abundant of the native compounds which 

 occur in the Hartz. [LEAD, in NAT. HIST. Div.] 



The characteristic properties of selenium are those of tinging the 

 flame of the blowpipe of a light blue colour, and emitting an acrid 

 vapour, when heated in the air, which has the peculiar smell of 

 decayed horse-radish. 



SELENIURETTED HYUHOGEN. [SELENIUM.] 



SELF-REGISTERING INSTRUMENTS. In the determination 

 of scientific data, it ia of the utmost importance that the instruments 

 of observation be as free from error as possible, or that the sources 

 and amount of error be accurately known. It is further necessary 

 that the means of observation be irreproachable, and, if possible, con- 

 tinuous. Where an instrument requires to have its indications recorded 

 night and day, a large staff of observers is required, and they are liable 

 to error, even when carefully trained to their work. [EQUATION, 

 PERSONAL.] The magnetic instruments at the Greenwich Observatory 

 were formerly observed every two hours ; but the results were of 

 course liable to error, and occasionally the magnetic variations were 

 too rapid and transient to be recorded^ Hence attempts and many 

 of them successful ones have been made from time to time to make 

 instruments of observation record their own results. We have seen 

 under ANEMOMETER the contrivances for registering the direction and 

 pressure of the wind, together with the amount of rain ; and under 

 BAROMETER some of the mechanical arrangements are stated for 

 making that instrument self-registering. The maximum and mini- 

 mum thermometers in ordinary use will be described under TIIERIIO- 

 IIETEH. Our object in this place is briefly to notice the vast improvement 

 that has token place of late years in consequence of the application of 

 photography to the purpose* of self-registration. A description of the 

 apparatus employed for the self-registration of the changes in position 

 of the declination magnet, of the horizontal force magnet, and of the 

 vertical force magnet, will show the nature of the arrangements. A 

 prepared photographic paper is wrapped round a cylinder, the axis of 

 which is placed parallel to the direction of movement to be registered, 

 and the cylinder is turned round at a uniform rate by clockwork. The 

 light U supplied by a gas-lamp furnished with a copper chimney, in 

 which is a small slit capable of adjustment by a screw. It ia ou the 

 breadth of this slit that the breadth of the register-line depends. This 

 light falls upon a concave speculum, which rests in a stirrup connected 

 with the magnet to be observed, so as to partake of all its angular 

 movements. The pencil of light is reflected from the mirror to a plano- 

 convex lens, placed near to, and parallel with, the axis of the cylinder, 

 which lens condenses the line of light to a definite spot of light on the 

 paper. The source of light being fixed, it in evident that the move- 

 ments of the spot of light will correspond with those of the magnet, to 



