284 



THE POPULAR EDUCATOK. 



choride by carbon at a high heat, or by sodium. It proves to 

 be most difficult of fusion, even more so than platinum. When 

 obtained by the latter process, it appears in crystals of the 

 first system. It is only of value on account of the colour of its 

 oxides and salts, which are used as pigments. 



The oxides of chromium are four : Chromous oxide (CrO) ; 

 chromic oxide (Cr 2 3 ) ; an intermediate oxide, which may be 

 considered as a compound of the CrO,Cr 2 O 3 ; and the only acid 

 oxide, chromic acid (CrO,). 



Chromous Oxide is only obtainable as a hydrate. When 

 caustic potash is added to a solution of chromous chloride, it 

 falls as a brown powder. 



Chromic Oxide is a sesquioxide (Cr 2 3 ). When ammonia is 

 added to a solution of chromic chloride, this oxide falls as a 

 hydrate. When heated to render it anhydrous, it becomes a 

 green powder, which is used as a paint. Chromic oxide forms 

 . two sets of salts one green, which is amorphous ; the other 

 violet, which crystallises. The emerald owes its colour to the 

 presence of this oxide. 



Chromic Acid (Cr0 3 ) may be obtained in many ways one is 

 by mixing four measures of the solution of potassium bichro- 

 mate with five of sulphuric acid. As the mixture cools, the 

 oxide separates out into crimson needles. It exhibits an acid 

 reaction, forming with bases chromates, which are yellow or red 

 salts. Neutral chromates are yellow, as potassic chromate 

 (K z O,Cr0 3 ). Potassic bichromate (KjO^CrO,,) is the salt 

 chiefly manufactured, and from which the various pigments are 

 derived. It exists in large red transparent four-sided tables. 



" Chrome Yellow " is lead cnromate (PbO,Cr0 3 ), and is pro- 

 cured by precipitation from a dilute solution of lead acetate by 

 a solution of potassic bichromate. When chromic acid is heated 

 with strong sulphuric acid, it gives off half its oxygen, thus 



2CrO s + SH^SOi = Cr a 3SO + 3H,O + 30. 



" This mixture affords one of the most powerful means of oxi- 

 dising carbon at comparatively low temperatures." (William- 

 son.) The chromic sulphate formed in this reaction is the 

 compound which forms with potassium and ammonium sulphates 

 a series of alums. 



A peculiar dark strongly-fuming liquid is obtained when a 

 mixture of a chromic salt, sodium chloride, and sulphuric acid 

 is heated. It comes off as a red vapour, and may be considered 

 as chromic acid, where one atom of the oxygen has been replaced 

 by its equivalent two atoms of chlorine. Hence its formula 

 is CrO 2 Cl 2 , and its name chloro-chromic acid. 



There are two known chlorides chromous chloride (CrCl 2 ), and 

 chromic chloride (Cr 2 Cl 9 ). The latter is obtained as violet- 

 coloured scales, when a current of dry chlorine is passed over an 

 intimate mixture of chromic oxide and charcoal heated to red- 

 ness. This variety is insoluble ; but it becomes green and loses 

 its insolubility when a very small quantity of the chromous chlo- 

 ride is added to it suspended in water, or the soluble chloride 

 is more readily obtained by boiling the solution of a chromate 

 with hydrochloric acid and alcohol. 



The chromous chloride is a white powder procured by heating 

 the chromic chloride in a current of hydrogen. The other salts 

 of this metal do not present any great interest. 



Chromic oxide imparts to glass or a borax bead a good green 

 colour. The salts of chromium give insoluble precipitates with 

 lead and silver compounds, of a yellow colour. All the chro- 

 mates, when treated with dilute sulphuric acid and a little 

 alcohol or sugar (organic matter), become reduced to the green 

 oxide of chromium. 



UEANITJM. 



SYMBOL, U COMBINING WEIGHT, 120. 



The chief ore of this rare metal is pitch-blende. The metal is 

 of a whitish colour, and does not oxidise when exposed to the alir ; 

 but if heated in air, it burns vividly. Two oxides are known, 

 and both are used in glass- staining. 



Uranous Oxide (UO), or the protoxide of this compound, ap- 

 pears analogous to chromous oxide. When heated in a current 

 of hydrogen, it refuses to give up its oxygen. If this oxide be 

 heated to bright redness, and then suddenly cooled, the intense 

 black is furnished with which porcelain is coloured and glass 

 stained black. 



Urania Oxide (U 2 O 3 ) is a yellow powder which is soluble in 

 mineral acids and ammonium carbonate. This peroxide imparts 

 to glass that peculiar yellow-greenish tint which characterises 

 uranium glass, which is also romrlirfcls for its fluorescence. 



ARSENIC. 

 SYMBOL, As COMBINING WEIGHT, 75. 



Arsenic is considered a non-metallic element by certain French 

 chemists, on account of its striking analogy to phosphorus and 

 nitrogen ; but seeing that it conducts electricity readily, and 

 possesses a metallic lustre, it is generally classed among the 

 metals. Probably its true place is between the classes, being 

 the "transition" element. Like phosphorus, its vapour offers 

 an exception to the rule. Its density, instead of being repre- 

 sented by its combining number, is just double, or 150. This 

 may be expressed by saying, that the volume occupied by an 

 atom of arsenic in its gaseous condition is just one-half that of 

 the other elements. The reason of this has not yet been dis- 

 covered. The metal occasionally is found native, but generally 

 as an alloy with iron, cobalt, nickel, etc. " Eealgar " is the sul- 

 phide, and " orpiment " the tersulphide, both of which are rare 

 minerals. The metal is chiefly obtained from mispickel, an 

 arsenical sulphide of iron. The fumes from the furnace are 

 conducted into chambers where arsenious acid (As 2 O 3 ) is con- 

 densed. When heated with charcoal, this oxide is reduced, and 

 the metal which vaporises is got by condensation. It has a 

 brilliant steel-like lustre, and is very brittle. It refuses to melt, 

 but readily volatilises, the vapours condensing on the first cold 

 surface they meet into rhombohedra of the metal. The vapour 

 of arsenic possesses a strong odour of garlic. When heated in 

 air, the metal burns with a blue flame into 



Arsenious Acid (As 2 3 ). This is the virulent poison " arsenic," 

 the wliite arsenic of the shops. It is obtained from the ore, as 

 already stated, as a crystalline white powder. When heated, it 

 fuses into a semi-transparent vitreous mass, which gradually 

 becomes opaque, like a piece of white porcelain. It is soluble 

 in hot hydrochloric acid or hot ammonia, and these solutions, as 

 they cool, deposit arsenic in transparent crystals. If the process 

 be watched in a dark room, light is emitted as each crystal is 

 formed. Arsenious acid is soluble in alkaline solutions, soluble 

 arsenites being formed. When to such a solution in potash 

 copper sulphate is added, the much-used pigment " Scheele's 

 green" is precipitated. 



When silver nitrate is added to the solution of potassium 

 arsenite, the yellow silver arsenite (3Ag,As0 3 ) falls. When a 

 solution of arsenious acid is acidulated with hydrochloric 

 acid, and sulphuretted hydrogen passed through it, the yellow 

 arsenious sulphide is precipitated. 



The best antidote for this violent poison is magnesia, which 

 forms with it an insoluble arsenite, and thus the poison passes 

 as a foreign substance through the body. Ferric oxide has the 

 same effect. 



Arsenic Acid, (As 2 O 5 ) is obtained by oxidising arsenious acid 

 by an excess of nitric acid, and evaporating to dryness in a 

 platinum vessel. It is more soluble than arsenious acid, but 

 does not volatilise when heated, being decomposed into the 

 lower oxide and oxygen. This acid, in forming arsenates, be- 

 haves as phosphoric acid, being tri-basic (H 3 AsOJ, and one or 

 two or all the atoms of the hydrogen may be replaced. Arsenic 

 may be readily distinguished from arsenious acid by the red- 

 brown precipitate it affords with silver nitrate. Its poisonous 

 properties are even more decided than those of arsenic. The 

 sulphides of arsenic have been mentioned. Realgar is made 

 artificially by heating together 198 parts of arsenious acid and 

 112 of sulphur. It is one of the ingredients in " ivhite Indian 

 fire," which is a mixture of seven parts of sulphur, two of 

 realgar, and twenty-four of saltpetre. Orpiment is the chief in- 

 gredient in King's yellow. There is a penta-sulphide also known. 

 Arsenic combines with chlorine, bromine, and sodium, forming 

 compounds of but little interest. 



The Tests for Arsenic. If a slip of clean copper-foil be im- 

 mersed in a solution of arsenious acid in hydrochloric acid, a 

 grey film of metallic arsenic is deposited on the copper. 



Marsh's Test is : Introduce the liquid suspected of contain- 

 ing the poison into a bottle in which hydrogen is being generated 

 in the usual way. Set fire to the escaping gas, and if the flame 

 burn blue, arsenic is present. When a cold surface is depressed 

 into the flame, a film of metallic arsenic is found upon it. 



The gas which thus burns is Arseniuretted Hydrogen (AsH 3 ), 

 which corresponds to NH 3 and PH 3 . It is capable of condensa- 

 tion, and at 40 Cent, becomes a liquid. The gas is absorbed 

 by copper sulphate, copper arsenide being precipitated. It is a 

 deadly poison. The inhaling of a bubble has been attended 



