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By the abstraction of one molecule of water from two molecules of these acids, still more complex acids would be derived, thus

10(HO).
IO(HO): - H.0=0 or H,12011. (4.)
10(HO),

10(HO).

10,(HO,
IO (HO)
10,(HO)
- H,0=0 or H,120g. (5.)

10,(HO)2 And from these two acids the following periodates may be regarded as being derived

(4.) 2141,011; Ba, 2011.
(5.) Ag,1,Og; Ca 1,09; Ba,1,Og.

HYPOIODOUS ACID AND HYPOIODITES. When an aqueous solution of iodine is added to either ammonia, potassium, or sodium hydroxides, lime-water or baryta-water, a colourless solution is obtained which possesses bleaching properties. The liquid is a dilute solution of the hypoiodite and iodide of the alkali used. Somewhat stronger solutions may be produced by adding small quantities of powdered iodine to the mixture

2KHO+12+Aq=KIO+KI+H,O+Ag. A dilute solution of the acid itself is obtained by shaking mercuric oxide with iodine and water (see Hypochlorous Acid, p. 373).

The solution of the alkaline hypoiodite obtained by the above reaction possesses well-marked bleaching properties. When freshly prepared it is without action upon starch, but is immediately decomposed by even so feeble an acid as carbonic acid, when the blue starch compound is at once formed.

A compound of iodine with lime, analogous to bleaching powder, has been obtained by shaking powdered iodine with milk of lime. The compound in the presence of water appears to behave in the same way as bleaching powder, yielding a solution of calcium hypoiodite and calcium iodide-

2Ca(OI)I=Ca(OH)2+Calz. On filtering the mixture a colourless liquid is obtained, which gives no reaction with starch, but which yields iodine when treated with an acid.

Neither the acid nor any of its salts has been isolated, being known only in dilute solution. The compounds are all extremely unstable, decomposing at the ordinary temperature in a few hours, and in a few minutes when the solutions are boiled; the salts passing into iodides and iodates-

3K10=2KI+KIO3, while the acid decomposes first into hydriodic and iodic acids, which then react upon each other with elimination of free iodine.

COMPOUNDS OF THE HALOGENS WITH EACH OTHER. Chlorine unites both with bromine and with iodine, and the two latter elements combine with each other.

(1.) Chlorine and Bromine.--Bromine monochloride. This substance is obtained as a reddish-yellow liquid, when chlorine gas is passed into bromine, The compound is believed to have the composition BrCi.

(2.) Chlorine and Iodine. - Iodine monochloride, ICI. When dry chlorine is passed over iodine, the latter rapidly melts, forming a dark reddish browni liquid, strongly resembling bromine in appearance. The liquid solidifies to a mass of red prismatic crystals, which melt at 25°. It is decomposed by water into iodic and hydrochloric acids, and iodine is liberated

51C1+34,0=HIO:+5HCI+213. lodine trichloride, ICly. This compound is formed by passing an excess of chlorine over iodine, or by passing chlorine through iodine monochloride. It is also formed when hydriodic acid is acted upon by an excess of chlorine

HI+2Cl,=HC1+ ICI:. Iodine trichloride is a yellow solid substance, crystallising in long brilliant needle-shaped crystals, which sublime at the ordinary temperature. When gently warmed it melts, at the same time dissociating into chlorine and the monochloride; on cooling, reunion takes place with the reformation of ICI

(3.) Bromine and Iodine. --Two compounds of these elements are believed to exist, viz., a crystalline solid and a deep-coloured liquid. Their composition is probably expressed by the formulæ, IBr and I Brz.

CHAPTER II

The ELEMENTS OF GROUP VI. (FAMILY B.)

Oxygen, O . . 16.00 Selenium, Se . . 79.1

Sulphur, S . : 32.06 l Tellurium, Te . . 127.6 The relation in which oxygen, the typical element, stands to the remaining members of the family is very similar to that between fluorine and the other halogens.

All the elements of this family unite with hydrogen, forming compounds of the type RH,

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but the hydride of oxygen stands apart from the others in many of its attributes. Thus at ordinary temperatures it is a colourless and odourless liquid, while the remaining compounds are all fætidsmelling and poisonous gases.

Sulphur, selenium, and tellurium each combines with oxygen, forming respectively SO3, Se03, and Te03, while none of these elements in a divalent capacity forms a similar compound ; that is to say, no such combinations are known as OS3, or O Sez, although amongst themselves they unite, forming Ses, and TeS..

Sulphur, selenium, and tellurium also unite with oxygen, forming dioxides, SO,, SeO.,, and Te0,, in which these elements are possibly tetravalent, in which case the constitution of the compounds will be represented thus, O=S=0); O=Se=0. We may, however, consider them as functioning in a divalent

O capacity, and regard the oxides as constituted thus, S 1 ; Sel,

in which case we may look upon ozone as being the corresponding oxygen compound, 002,0 T.

All the elements of this family combine with chlorine, producing compounds having the following composition :

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Oxygen again differs from the other members by alone forming a compound of the type, R.CI. This element also shows no tendency to function with a higher atomicity than that of a divalent; while the others unite with four atoms of the halogen, thereby exhibiting their tetravalent nature.

The members of this family pass by a regular gradation from the strongly electro-negative, gaseous, non-metal oxygen to the feebly negative and slightly basic element tellurium, which possesses many of the properties of a true metal. Selenium and tellurium are both elements which lie very close to that ill-defined boundary between the metals and non-metals, and are on this account sometimes termed metalloids. In tellurous oxide, Te0., we have a compound which is both an acid-forming and a salt-forming oxide, its acidic and basic properties being nearly equally balanced. Thus, it replaces hydrogen in sulphuric acid, forming tellurium sulphate. Te(SO22; and it also unites with water, forming tellurous acid, H2TeO3, corresponding to sulphurous acid, H,SO3.

Of the four elements of this family, oxygen is by far the most abundant, both in combination and in the free state ; sulphur is more plentiful than the other two, and tellurium occurs in the smallest quantity.

The element oxygen has already been treated in Part II.

SULPHUR. Symbol, S. Atomic weight = 32.06. Molecular weight=64.12. Occurrence.-In the free state this element occurs chiefly in volcanic districts. In Italy and Sicily large quantities of natione sulphur are found, which have long been the most important European sources of this substance. Large deposits are to be met with in Transylvania and in Iceland, and it also occurs in beds, often of great thickness, in parts of China, India, California, and the Yellowstone district of the Rocky Mountains. These natural deposits are sometimes found stratified with beds of clay or rock, but they often occur as what are known as “living” beds, in which the sulphur is continuously being formed as the result of chemical decompositions which are at present at work. Such a "living" sulphur bed is known as a solfatara, and, as in the case of the Iceland deposits, they are usually found associated with geysers, fumaroles, and other signs of volcanic action.

In combination with hydrogen, sulphur occurs as sulphuretted hydrogen. Enormous quantities of sulphur are found combined with various metals, constituting the important class of substances known as sulphides; as, for example, galena, or lead sulphide, PbS; zinc blende, or zinc sulphide, ZnS ; pyrites, or iron sulphide, FeS2 ; copper pyrites, or copper iron sulphide, Cu, Fe254; stibnite, or antimony sulphide, Sb2S3 ; cinnabar, or mercury sulphide, HgS.

In combination with metals and oxygen, sulphur occurs in sulphates, such as gypsum, CaSO4,2H20; heavy spar, BaSO4; kieserite, MgSO4,H20.

Modes of Formation.-(1.) Sulphur is formed when sulphuretted hydrogen is brought in contact with sulphur dioxide ; the two gases mutually decompose one another with the formation of water and the precipitation of sulphur

2H2S+S02=2H,0+3S.

(2.) It is also produced when sulphuretted hydrogen is burnt with an insufficient supply of air--

H.S+0=H2O+S. This reaction probably takes place in two stages, a portion of the sulphuretted hydrogen burning to sulphur dioxide, and 'this then reacting upon a further quantity of sulphuretted hydrogen, thus

(a) H.S+30=H2O+SO.2.

(6) 2H.S+SO,=2H2O +35. It is supposed that some of the free sulphur found in volcanic regions has been produced by this action of these two gases upon one another.

Extraction of Sulphur from Native Sulphur.-- Natural sulphur is always more or less mixed with carthy or mineral

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