VAPOR, in physics, an aeriform fluid into which some volatile substance is changed by the action of heat. Vapor is essentially the same as gas, but the word vapor is conventionally limited to the gaseous state of a body which is liquid or solid at ordinary temperatures, while the term gas is applied to aeri form bodies which are in that rarefied state at ordinary temperatures. Thus we speak of hydrogen gas, but of watery vapors. Vapors, like gases, have a cer tain elastic force, by which they exert a pressure on every part of any vessel in which they are inclosed. Vapors are formed instantly in a vacuum; in the at mosphere they are generated more slow ly. When not saturated they exactly re semble gases in their action; when satu rated and in contact with the liquid by which they were generated, they can neither be compressed nor expanded, but remain constant, both in their elastic force and in their density. Vapors of different composition vary in density. Thus if atmospheric air be taken as unity, the vapor of water=0.6235, that of alcohol 1.6138, that of sulphur 6.6542, and that of mercury 6.9760.
VAR, a department in the extreme S. E. of France; area, 2,349 square miles; pop. about 340,000. The depart ment receives its name from the Var river, which formerly served as its boun dary on the E., but which, since the arrondissement of Grasse was taken from Var and added to the Alpes Mari times, now belongs entirely to the lat ter. Var is well watered by a great number of streams, of which the chief are the Gapau, Argens, and Bianson. In the N. and N. E. it is mountainous, being traversed by a branch of the Alpes de Provence, called the Monts de l'Esterel. Between the mountains and the water courses are many very fertile valleys. The climate, tempered by the altitude of the surface, is pleasant. Fruits of all kinds are here cultivated with remarkable success; tobacco is grown, and much wine is produced. The department abounds in minerals, and carries on an active commerce. It is divided into the three arrondissements of Draguignan, Brignoles, and Toulon. Capital Draguignan.