10 



VANDALS. 



VAPOUR. 



SCO 



quantity it exists in nearly all clays, but occurs in abundance in a lead 

 ore (vaiiadiate of lead) found at Wanlockhead in Scotland, Zimapan in 

 Mexico, and recently in Chili 



Jjy reducing vanadic anhydride with potassium, and digesting the 

 product in water, vanadium ia obtained as a brilliant, metallic powder ; 

 soluble in nitric acid or aqua regia, but unacted upon by boiling sul- 

 phuric, hydrochloric, or nitric acids. 

 The equivalent of vanadium is 68'46. 



tdium and o.cij'jcn combine in three proportions. The protoxide 

 (VO) has the appearance of graphite, and is formed when hydrogen is 

 passed over heated vanadic acid. The binwide (VO,,) ia a black powder 

 produced when the protoxide is heated in the air; it combines with 

 acids to form salts, which have a blue colour. The teroxide or tanadic 

 anhydride (V0 3 ) is a brownish-red powder that remains on heating 

 bivanadiate of ammonia. It fuses at a red heat, and is slightly soluble 

 in water, to which it communicates a yellow tint and an acid reaction. 

 Vanadic acid (2 HO, VO,) falls as a precipitate when nitric acid is added 

 to a hot solution of a bivanadiate ; in appearance it much resembles 

 hydrated oxide of iron. Dried over sulphuric acid it loses an equivalent 

 of water, and protohydrate (HO, V0 3 ) remains. 



Yanadiatct are obtained on decomposing bivanadiate of ammonia by 

 a chloride. They contain 



Eiranadiate of eo !a . 



itrontia 



lime . 



magnesia 



Baryta fait . . 



, :.-ao, svo, +ono 



. SrO, 2VO, -t-9aO 

 . CaO, SVO, -f 9110 

 . MgO, SVOj + SHO 

 , 3BaO, 5VO 3 +19IIO 



To produce bivanadiate of ammonia Hauer recommend* that the 

 crude vanadium compound be calcined, the residue digested in water, 

 the insoluble portion fused with nitrate of potash, the resulting mass 

 digested in water, the solution concentrated, and vanadiate of ammonia 

 precipitated by adding excess of chloride of ammonium ; it is purified 

 by repeated crystallisations from water containing acetic acid. 



Two cliiorida (VCL, and VCI,),as well as sulphides, bromides, iodides, 

 and fluorides of vanadium, have been obtained. 



Tuti fi/r Vanadium. By reducing agents, such as sulphide of hydro- 

 gen, or a boiling mixture of sulphuric acid with alcohol or sugar, 

 vanadiates yield beautiful blue solutions. This reaction distinguishes 

 them from chromates, which under the same circumstances give green 



VANDALS. [TnuTOMC NATIONS.] 



VANILLA, [VAKILLA, Anniatiru, in NAT. HIST. Drv.] is a native 

 of Brazil. The fruit is the only part of the plant that is used. It has 

 a balsamic odour, and a warm agreeable flavour. For these properties 

 it is indebted to a peculiar volatile oil, and to a considerable quantity 

 of benzoic acid. The fruit is gathered when it gets yellow, and it is 

 first allowed to ferment for two or three days : it ia then laid in the 

 sun to dry, and when about half dried it is rubbed over with the oil of 

 cocoa : it is again exposed to the sun to dry, and oiled again a second 

 tii:i>-. The fruit is then collected in small bundles, and wrapped up in 

 the leaves of the Indian reed, and sold to the Europeans. It is used to 

 flavour cho' 



VANISH (Mathematics). A quantity Is said to vanish, or to become 

 evanescent, when ifcs arithmetical value is nothing, or denoted by 0. 

 When the evanescent quantity is only a part of another, there is 

 seldom or never any more difficulty about the case in which it 

 vanishes than about that in which it takes any other specified value : 

 but when the whole of what is under consideration vanishes, any or 

 nil of those views may be required to render this case intelligible 

 which are explained in NOTHING; ISFIHITE; LIMITS; RATIOS, PRIME 

 AND ULTIMATE ; &c. And in particular the phrase of two quantities 

 vanishing in a certain ratio is to be referred to the last of the articles 

 cited. When a value given to a letter makes an expression vanish, or 

 reduces it to 0, it would bo very convenient to say that the given value 

 nMifla, and is a nullifier of the expression : bat this language is 

 not UHI- !. 



VANISHING FRACTIONS. [FRACTIOUS, VAWTSHWO.] 



VANISHING POINT, LINE, Ac. [PERSPECTIVE.] 



VAPORISATION is the process by which a liquid on being suffi- 

 ciently heated passes off in the form of vapour. It differs from 

 EVAPORATION in being generally an artificial and a quicker process, 

 while the latter is spontaneous. 



VAPOUlt. There are many substances, both fluid and solid, which 

 when exposed to the air, or to the more powerful agency of heat, are 

 gradually but totally dissipated, owing to their particles assuming the 

 (!t"it<; of vapour by what is termed spontaneous craporation. A vapour, 

 then, consists of ponderable matter combined with sufficient specific 

 heat to enable it to retain its aeriform existence: we have already 

 [GAS] given a similar definition of a gas. The question, then, natu- 

 rally arises, In what do vapours differ from gases ? The answer is, 

 that the difference is a conventional one, being of degree only, and not 

 of kind : thus, when atmospheric air containing, as it always does, the 

 rapour of water, is suddenly cooled by exposure to a colder substance, 

 the water which it contained in the state of invisible vapour is depo- 

 sited in the state of palpable water on the colder body ; we say then 

 aqueous vapour or the vapour of water, and not aqueous gas. No 



similar change is produced, by this abstraction of heat, in the form of 

 the constituents of the air, and they are therefore termed gaseous 

 bodies or gases. The difference, however, we repeat, is one of degree 

 only ; for many gaseous bodies which had been, not many years since, 

 considered as permanently elastic as atmospheric air, have been shown 

 by the important investigations of Dr. Faraday to be reducible to 

 liquids [GASES, LIQUEF ACTION OP]; and additional experiments have 

 even shown that carbonic acid gas, which requires a pressure of 35 

 atmospheres to render it fluid, may by particular management be con- 

 verted into a solid. [CARBONIC ACID.] 



A practical difference between a vapour and a gas is illustrated by 

 the use of the vapour of water, and its subsequent condensation, as a 

 motive-power in the steam-engine. [STEAM and STEAM-ENGINE.] No 

 known gaseous body could be employed with the same advantage, 

 owing to the great degree of pressure and cold required for its con- 

 densation. Nor could the vapour of any other liquid than water be so 

 profitably employed as a prime mover, for the reason given under 

 LATEST HEAT. 



It was formerly supposed that the air dissolved vapour and held it 

 in solution as water holds sugar or salt. This was the theory of Lo 

 Roi, propounded in some otherwise ingenious papers, ' Sur 1'Elevatiou 

 et la Suspension de 1'Eau dans 1'Air et sur la RoseSe.' (' Mem. Acad. 

 Royale des Sciences,' 1752 ; and also in a separate volume, ' Melanges 

 de Physique et de Medicine,' 1771.) This theory, however, was com- 

 pletely demolished by Dalton at the commencement of the present 

 century (' Manchester Memoirs '), who showed that a vapour forms 

 much more easily in a vacuum, and that the pressure of air retards and 

 obstructs the evaporation of a liquid. Dalton's apparatus consisted 

 chiefly of two barometers placed side by side. A drop of a given 

 liquid was sent up into the vacuum of one of them, when it becamo 

 converted into a transparent vapour, and, exerting a pressure on tho 

 mercurial column, lowered the mercury in the tube. The amount of 

 this depression was measured by comparison with the adjacent baro- 

 meter, and the elasticity of the vapour was expressed not by that 

 depression, but by an equal quantity of mercury which the vapour 

 would support. Thus, if the summit of the column of the mercury 

 containing the vapour stood half an inch below the mercury in the 

 adjacent barometer, the pressure of the vapour would be such as 

 would support a column of mercury half an inch in height. In esti- 

 mating this pressure, sufficient liquid must be sent into the vacuum to 

 saturate it, and it is then found that the elasticity of the vapour is 

 directly as the temperature. By surrounding the tube with some 

 vessel capable of containing hot water, with a thermometer for indi- 

 cating temperature, the temperature of the vapour can be known, and 

 the relation between its temperature, pressure, and density ascertained. 

 [EvAroBATloic.] Various forms of apparatus have been contrived by 

 Dalton, Gay-Lussac, Arago, and Dulong, and by Regnault, for deter- 

 mining the pressures and densities of vapours, and obtaining such 

 results as are given in the table under STEAM and STEAM-ENGINE. 



The specific gravities of vapours, like those of gases, arc referred to 

 air as a standard at the temperature of 32 and a pressure of 30 inches; 

 or the density of a vapour may merely express the ratio of a given 

 volume to an equal volume of air of the same temperature and pressure. 

 In the case of steam above 250 this ratio is invariable, and indeed the 

 ratio of tho densities is constant for the vapour of most liquids above 

 their boiling points. These ratios have been thus determined by Gay- 

 Luasac : 



Air 10,000 



Vapour of water 6,233 



alcohol 10,138 



sulphuric ether . . . . 25,800 



sulphnrct of carbon . . . . 20,447 



essence of turpentine . . . . 60,130 



mercury ...... G,9"0 



,, iodine . 8,716 



V/hen a gas and a vapour which do not act chemically on each other 

 are inclosed in the same space, they will exert separately on the sidca 

 of the vessel the same pressures that each would produce if it occupied 

 the same space in the absence of tho other, so that the total pressure of 

 the mixture is equal to the sum of the separate pressures. 



When vapour receives a supply of heat after it has been separated 

 from the liquid it is called super-heated vapour. Such vapour, uuliko 

 ordinary vapour, may lose a portion of its heat, and still the whole of 

 it continue to be vapour. If, after a vapour has been raised from a 

 liquid, it be compressed into a smaller space, its temperature will rise, 

 and if expanded it will fall; but the temperature, pressure, and volume 

 will always be such as the vapour would have had if it had been raised 

 directly from the liquid at such temperature and pressure. Thus, 

 vapour raised from water at 68* has a volume 58-224 times greater 

 than tho water that produced it. If this vapour be separated from the 

 water, and Ita volume be compressed until it is only 1696 times that of 

 the water. Its temperature will rise to 212, or that which it would 

 have had U directly raised from the water under the increased 

 pressure. 



Evaporation, both spontaneous and artificial, and especially the 

 latter, is employed In numerous manufacturing and chemical processes. 

 When, for example, common salt ia prepared from sea-water, it is 

 exposed in the first instance to the air in shallow clay pits, by which 



