561 



ARSENIC. 



ARSENIC. 



62 



in atmospheric air, when, being very combustible, it burns and com- 

 bines with oxygen ; the white vapour of arsenious acid formed, speedily 

 condenses, and frequently in the form of the regular octahedron ; this 

 acid may also be procured by heating the metal in very dilute nitric 

 acid, which being decomposed yields oxygen. These processes are, 

 however, needless, for arsenious acid is met with abundantly, and very 

 pure, as an article of commerce ; being formed and volatilised during 

 the roasting of cobalt ores, it is first condensed in an impure state, and 

 purified by a second sublimation in an iron vessel. Arsenious acid 

 (oxide of arsenic, the white arsenic of the shops, and of the ' London 

 Pharmacopoeia'), has the following properties : it occurs in compact 

 masses of various sizes, which are externally colourless and opaque, but 

 internally, when recently broken, frequently yellowish and transparent, 

 and of a glassy appearance and fracture ; by exposure to the air the 

 transparency is lost ; the density of the opaque kind is 3706, and that 

 of the glassy 3'699. Arsenious acid is volatilised at 380 Fahrenheit ; 

 the vapour has not the garlic smell, like that of metallic arsenic. 

 According to Dr. Christison, arsenious acid has little or no taste. It is 

 well known as a most virulent poison. Arsenious acid is soluble in 

 water ; at about 60 Fahrenheit it dissolves about l-50th of ite weight, 

 and when boiling, nearly l-13th; on cooling to 60, a considerable 

 portion is deposited in octahedral crystals, so that only about 3-8ths of 

 that taken up remains dissolved; the solution reddens litmus paper 

 but slightly, and though it acts feebly as an acid in this respect, and 

 does not decompose the alkaline carbonates when cold, yet it expels 

 their carbonic acid when they are heated together in solution. The 

 nature of the saline compounds will be presently mentioned. 



In order to avoid the danger of accidentally poisoning by arsenic, or 

 any colourless preparation of arsenic, and to render its use more difficult 

 for intentional poisoning, an Act was passed regulating the sale of such 

 substances, in the session of parliament 1851. By this Act it is 

 required that, except when sold for agricultural or other purposes in 

 quantities of not less than ten pounds, or to be used in medicine by a 

 qualified practitioner, all arsenious acid is to be mixed and coloured 

 with soot or indigo ; that all persons selling arsenic are to keep a book 

 in which the name, place of abode, condition or occupation of the pur- 

 chaser, the quantity sold, and the purpose for which it is required, shall 

 be entered, certified by the signatures of the seller and the purchaser, 

 or the referee of the purchaser, whenever he U not personally known 

 to the seller. Any infraction of these provisions subjects the offender 

 to penalties not exceeding 201. for each offence, to be levied by two 

 justices on summary conviction. 



Arsenic acid (As0 5 ) exists in nature much more commonly than the 

 arsenious acid ; sometimes it is found combined with lime, and 

 frequently with various metallic oxides, as those of copper, iron, and 

 lead ; the areeniates of copper constitute, indeed, a most beautiful and 

 extensive variety of the ores of that metal. 



Arsenic acid may be formed artificially by heating either the metal or 

 arsenious acid in nitric acid, or, which is preferred, in a mixture of 

 nitric and hydrochloric acid ; the mixture is to be distilled in a glass 

 retort until it has acquired the consistence of a syrup ; afterwards it is 

 to be heated nearly to redness in a platina crucible, until all the nitric 

 acid is expelled. Arsenic acid thus prepared is of a milk-white colour ; it 

 contains no water, but when exposed to the air attracts moisture until 

 a solution of specific gravity 1'935 is obtained ; when water ia added to 

 the anhydrous acid it dissolves only a part of it for a long time, 

 leaving a white powder, which i, however, eventually taken up ; by 

 evaporation a syrupy liquid of specific gravity 2'55 is obtained, which, 

 when concentrated till the temperature rises to 248, begins to deposit 

 Bclid matter. The anhydrous acid has not a very strong taste, but 

 the aqueous solution U extremely sour, acts strongly as an acid on 

 litmus paper, and decomposes alkaline carbonates when cold. It is 

 extremely poisonous. Its salts, termed arseniates, will be mentioned 

 hereafter. 



Artenuretted hydrogen (AsH,) Arsenic is one of the few metals 

 which form permanent compounds with hydrogen. Arsenuretted 

 hydrogen may be prepared by fusing equal weights of arsenic and 

 zinc, and dissolving the alloy in hydrochloric acid ; the gas may be 

 received in air jars filled with and inverted in water, in which it is 

 insoluble. This compound is gaseous at the usual temperature of the 

 air,: but when subjected to intense cold, it is condensed into a limpid 

 liquid resembling ether. The gas has an extremely fetid smell ; its 

 specific gravity is 2'695 ; it is fatal to animals when it forms only 

 l-10th of the air which they breathe. When exposed to atmospheric 

 air, it is decomposed ; metallic arsenic, mixed with some arsenious 

 acid, is deposited, and water is formed. It is composed of 6 volumes 

 of hydrogen gas, and 1 of the vapour of arsenic, condensed into 4 

 volumes. 



There is also a solid compound of arsenic and hydrogen ; it is 

 obtained by employing arsenic as the negative electrode, when water 

 is decomposed by electricity ; the hydrogen of the water, instead of 

 being evolved, combines with the arsenic, and the compound is 

 detached from the metal in brown-coloured flocks. Its composition 

 has not been clearly ascertained. 



Terchloride of arsenic (AgCl,). When arsenic in powder is thrown 

 into chlorine gas, it burns, owing to the heat attending the combina- 

 tion ; when also a mixture of 1 part of arsenic and 6 parts of perchloride 

 of mercury is distilled, a fuming colourless liquid condenses in the 



ABTS XSO BCI. DIV. VOL. I. 



receiver; the product in both cases is terchloride of arsenic. It is 

 very volatile, and does not become solid at a low temperature. If 

 water and chloride of arsenic are mixed, both are decomposed, and 

 araenious and hydrochloric acids are formed. When heated, chloride 

 of arsenic dissolves sulphur and phosphorus, but they separate on 

 cooling. 



Another method of preparing chloride of arsenic is, to put 1 part of 

 arsenious acid and 12 parts of sulphuric acid into a retort, heat the 

 mixture nearly to 212, and then gradually add small fragments of 

 fused common salt ; pure chloride of arsenic, which is to be condensed 

 by artificial cold, passes over into the receiver. A little water frequently 

 comes over with the chloride towards the end of the operation, and 

 this hydrated chloride does not mix with, but floats on, the anhydrous 

 chloride first distilled. 



Arsenic does not appear to unite with carbon ; it combines with 

 bromine, iodine, fluorine, selenium, and phosphorus ; but the resulting 

 compounds are not important. 



Compounds of sulphur and arsenic. Arsenic and sulphur may be made 

 to combine in several different proportions ; two of these sulphurets, 

 and the more important, exist in nature, and these only will be 

 described at any length. The first is the red sulphuret of arsenic, 

 commonly called realger (AsS,) ; this is found native in several parts of 

 Europe, and sometimes crystallised. It is of a deep-red colour, brittle, 

 easily reduced to powder, inodorous, tasteless, and insoluble in water : 

 its specific gravity is about 3'338. It may be artificially formed by 

 melting a mixture of arsenic and sulphur in a covered crucible, or the 

 arsenious or arsenic acid may be used. In the latter cases, sulphurous 

 acid is formed and evolved, owing to the oxygen of the acid combining 

 with a portion of the sulphur. In close vessels, it sublimes unchanged. 

 It appears to be poisonous, but less so than arsenious acid. It is used 

 as a pigment. 



The second is the yellow sulphuret of arsenic (Sulpharmnious acid, 

 AsS,) usually called orpiment. This sulphuret is also a natural product, 

 occurring rarely crystallised : it is commonly composed of thin plates, 

 which are of a very fine yellow colour, and flexible to a considerable 

 degree : its specific gravity is 3'452. It is insoluble in water, and 

 inodorous. Acids do not dissolve it, but nitric acid and chlorine decom- 

 pose it. When heated in close vessels, it. melts, and then sublimes ; 

 when heated in the air, it burns with a pale blue flame, and gives a 

 white vapour, and an odour of sulphurous acid. It may be formed 

 artificially by passing a current of sulphuretted hydrogen gas into an 

 acid solution of arsenious acid. It is used as a pigment, and is the 

 colouring ingredient of king's yellow. 



The other sulphurate of arsenic are unimportant. 



Arsenic and metal* in general combine with great facility : those 

 which are malleable it renders brittle, and those which are difficult to 

 melt, it renders fusible. 



The combination of arsenic with potassium and sodium is attended 

 with the disengagement of much heat. The resulting arseniurets are 

 decomposed by water, the potassium and sodium are oxidised, while 

 the hydrogen of the water converts the arsenic partly into gaseous 

 arsenuretted hydrogen, and partly into the brown arseniuret of hydro- 

 gen already noticed. 



The metallic arseniurets are not of sufficient importance to require a 

 more minute description. 



We have now to notice the salts that contain the arsenious and 

 arsenic acid, and which are termed respectively arsenites and arsmiates. 



Arsenite of ammonia (NH,OAsO,) may be prepared by dissolving 

 arsenious acid in solution of ammonia. It cannot be obtained in a 

 solid form, for by evaporation the salt is decomposed, ammonia is 

 evolved, and octahedral crystals are obtained, which are mere arsenious 

 acid without a trace of ammonia. 



Arsenite of potash (2KO,AsO,) is procured by digesting the acid in a 

 solution of the alkali. By evaporation, a saline mass is left, but no 

 crystals of the salt are formed. This compound is employed in the 

 preparation of arsenite of copper, sometimes called Scheete's green. It 

 is also the basis of the liquor arsenicalie of the ' London Pharmacopoeia 

 Two other arsenites of potash have also been formed. 



Arseniff of soda (2NaO,AsO,) is prepared as the last mentioned. By 

 evaporation, a viscid mass is obtained ; and when the evaporation has 

 been continued till the solution has acquired the consistence of a syrup, 

 small granular crystals are obtained as it cools. Two other arsenites of 

 soda are also known. 



Arsenite of lime (2CaO, AsO,) may be readily procured by mixing an 

 aqueous solution of the acid with lime water; the arsenite being 

 nearly insoluble in water, is precipitated in the state of a white 

 powder : it contains water, is soluble in acids, and even in some saline 

 solutions. It is poisonous. 



Arsenite of baryta (2BaO, AsO,) is a white powder, slightly soluble 

 in water. 



Arsenite of strontia (2SrO, AsO,) is soluble in water. 



Artenite of copper (CuO, AsO, ?). This compound was first prepared 

 by Scheele, and by him proposed as a pigment ; and it has been long 

 and extensively used as such, under the name of Scheele 's green. It is 

 formed by adding a solution of arsenite of potash to one of sulphate of 

 copper (blue vitriol). By double decomposition, arsenite of copper is 

 formed, and is precipitated of a fine green colour. The exact com- 

 position has not been determined : indeed, it is probable that more than 



o o 



