Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/510

498 CHEMISTRY [SULPHUR GROUP. Reaction. Units of heat. Remarks. I.,0 . 44,860 ( Formation of the anhy dride from its ele I, Cv. Aa ... 43,070 ments. Do. in aqueous solution. I 0* Aa . - 1,790 Heat of dissolution of r2 I 0, H 57,880 Formation of the acid 03 o I O, H Aq . 55,710 Do. in aqueous solution. T3 O IO..H An - 2,170 SHeat of dissolution of LO* HoO + 2,540 ( Combination of the an- IHAq O 3 42,540 Oxidation of dissolved .KOHAq,I0 3 HAq... f T 0,, H. 13,810 185,780 Heat of neutralization. ( Formation from the ICL1L Aa - 1,380 ( elements. Heat of dissolution. I (X H K Aa 184,400 Formation in aqueous TJ ? I 0, H . Aa 47,680 solution. Ditto. _o IHAq, 0,.., 34,510 ( Oxidation of dissolved o "C o PH !, , O? , Aq..., 27,000 ( hydriodic acid. Formation from the elements in aqueous I0 6 H 5 Aq,KOHAq I0 6 H 5 Aq,2KOHAq 5,150 ) 26,590 J solution. Heat of neutralization. It will be noticed that the amounts of heat developed in the formation of these two acids in aqueous solution from their elements are very simply related ; thus (I,,0 6 ,H 2 , Aq) = 11 1,420 heat-units or 3x37,140 (1 , G , H 5 , Aq) = 184,400 5 x 36,880 Again, both acids are easily decomposed by hydriodic acid, yielding water and iodine, and a similar simple rela tion obtains between the amounts of heat developed in the two reactions (LO 6 H 2 Aq, 10HIAq) = 167,040 heat-units, or 5 x 33,408 (I0 H 6 Aq,7HIAq) =133,770 ,,4x33,442 The analogous decomposition of chloric acid by hydro chloric acid would involve the absorption of 30,920 units of heat, whilst that of bromic acid by hydrobromic acid would be attended with the development of 101,520 units of heat; the difference between these numbers, 132,440, is almost equal to the amount evolved in the decomposition of periodic acid, and on comparing the several reactions it will be evident that the differences between the amounts of heat evolved are approximately simple multiples of a common constant ; thus Chloric acid Bromic ,, lodic , , Periodic 30,920 101,520 167,040 133,770 Differences. 132,440 65,520 33,270 The differences are thus to each other as 4:2:1. The meaning of this remarkable relation is at present unknown. Without further description, it will be obvious that whilst chlorine, bromine, and iodine and their compounds exhibit very considerable general resemblance, yet there are many most important differences between them ; and also that chlorine is much more closely related to bromine than is the latter to iodine. SULPHUR SELENIUM TELLURIUM. Sulphur, Symbol, S ; Atomic wt, 31 98. Selenium, ,, Se ; ,, 79. Tellurium, Te ; 128. These three elements are usually classed together, since most of their compounds with other elements are of the same type. In some respects they resemble each other closely, but in others they differ very widely, selenium being much more closely related to sulphur on the one hand than to tellurium on the other. Sulphur occurs native in most volcanic districts ; it is also found abundantly in combination with metals, such as copper, iron, lead, and zinc, and it is widely distributed in the form of sulphates of barium, calcium, magnesium, and strontium ; it is a necessary constituent of animal and vegetable tissues. Selenium is of rare occurrence, and is always met with in combination with other elements ; it is frequently asso ciated in minute proportions with sulphur in its ores. Tellurium is also a rare element ; it is occasionally found native, but generally in combination with various metals, such as bismuth, copper, lead, gold, or silver, and it is usually accompanied by selenium. Sulphur is a very brittle solid of a lemon-yellow colour, insoluble in water, and therefore tasteless. It is a bad conductor of heat, and a non-conductor of electricity. Its specific gravity in the native crystalline state is 2 - 05. Sulphur melts at 115 C., and at 120 C. is converted into a perfectly limpid pale yellow liquid ; but as it is gradu ally heated it becomes darker and more viscid until, at 200-240 C., it is so thick that the vessel in which it is contained may be inverted without any outflow taking place. At 250-300 C. it again liquefies, but does not become so fluid as when first melted, and at 440 C. it boils, yielding an orange-coloured vapour. The density of sulphur vapour at 1000 C. is 32 times as great as that of hydrogen at the same temperature ; so that as the atomic weight of sulphur is 32, the molecules of sulphur at this temperature are diatomic ; but at about 500 C. its density is three times as great, so that at this temperature its molecules must be regarded as hexa- tomic. Selenium is a deep brown-coloured brittle solid, of the specific gravity 4 5 in the crystalline state; like sulphur, it is insoluble in water. It boils at a temperature below a red heat, and gives off a deep yellow vapour, the density of which at about 1400 C. is such as to show that it con sists of diatomic molecules, but at lower temperatures, like sulphur, its molecule is more complex. Tellurium is much like a metal in appearance, resemb ling bismuth in colour ; it is very brittle ; its specific gra vity is 6 1 to 6 33, and it conducts heat and electricity, though not very readily. It melts at about 500 C., and at a high temperature is converted into a yellow vapour, the density of which at 1440 C. is such as to show that the molecule of tellurium at this temperature is diatomic. No tendency has hitherto been observed on the part of tellurium to form a more complex molecule in the manner characteristic of selenium and sulphur ; and it is noteworthy also that, whereas the density of sulphur vapour at temperatures not far removed from its boiling point is three times as great as at much higher temperatures, the density of selenium vapour under similar circum stances is only about one and a half times as great as at about 1400 C. Sulphur is chiefly remarkable for the great number of olio- tropic forms in which it can exist. Thus, native sulphur is found crystallized in rhombic octahedra of the specific gra vity 2 05. This variety melts at 114 5 C. ; it is readily soluble in carbon disulphide, the sulphide of chlorine, S 2 C1 2 , benzene, turpentine, &c., and sulphur is usually deposited in this form on spontaneous evaporation of its solution in carbon disulphide; it is, in fact, the stable form into which all other allotropes tend to change. When, how ever, melted sulphur is allowed to cool slowly it crystal

lizes in transparent, yellowish-brown, monoclinic prisms, of