VIR 



The line which separates the numerator 

 from the denominator of a fraction may 

 be regarded as a sort of vinculum, 

 corresponding, in fact, in Division to 

 the bracket in Multiplication. Thus, 



^ r— — i^tiplies that the whole quan- 

 tity a + 6 — c is to be divided by 5. 



VINDEMIA'TRIX. Prcevindemia- 

 trix. A star of the third magnitude in 

 the constellation Virgo ; it took its name 

 from the vintage. 



VINE. Viticula. A stem which trails 

 along the ground without rooting, or 

 entangles itself with other plants, to 

 which it adheres by means of its ten- 

 drils, as in the vine and the cucumber. 

 The term is now seldom used. De 

 Candolle refers this kind of stem to the 

 sarmentum or runner, from which, how- 

 ever, it essentially differs in its not root- 

 ing. 



VINEGAR. Acetic acid. A hydrated 

 acid derived by the action of air upon 

 alcoholic liquors, as wine and beer ; but 

 the remarkable discovery of Dr. J. Davy, 

 that platinum black in contact with alco- 

 hol became incandescent, and gave rise 

 to acetic acid, first led Dobereiner to the 

 discovery that alcohol, by absorbing oxy- 

 gen, gives rise to water and acetic acid, 

 without disengaging carbonic acid. 



Wood vinegar, or pyroligneous acid, is 

 prepared on a large scale by the distilla- 

 ti^i of wood, generally that of oak cop- 

 pice deprived of the bark, which is used 

 in tanning. 



VIOLA'CE;E (viola, a violet). The 

 Violet tribe of Dicotyledonous plants. 

 Herbs with leaves usually alternate; 

 flowers polypetalous ; petals hypogynous ; 

 stamens alternate with the petals ; ova- 

 rium 1 -celled, many-seeded. 



VI'PERID^. The Venom-snakes, in- 

 cluding the varieties of the vipera or 

 adder, the naia, the rattle-snake, the 

 horn-snake, &c. 



VIRGATE {virga, a twig). A virgate 

 stem differs from a vimineous stem only 

 in being less flexible. See Vimen. 



VIRGO. The Virgin; the sixth of 

 the zodiacal constellations, consisting of 

 110 stars, the principal of which is Spica 

 Virginis. It denotes the third month of 

 winter, extending from the 20th of 

 February to the 20th of March. In the 

 Egyptian zodiacs, famenoth, or the fruit- 

 ' ful woman, holds an ear of corn in her 

 hand. The Greeks were led into the 

 error of calling this sign irapdevo^, in 

 consequence of the Egyptian word sig- 

 368 



VIS 



nifying 'endowed with beauty;' but it 

 also involves the idea of fruitfulness. 



VIRGULTUM. The name sometimes 

 given to a young slender branch of a 

 tree or shrub. 



VIRTUAL FOCUS. A term in Optics 

 denoting the point from which rays of 

 light, having been rendered divergent by 

 reflection or refraction, appear to issue. 



VIRTUAL VELOCITY. The velo- 

 city which a body in a state of equi- 

 librium would actually acquire during 

 the first instant of its motion, in case of 

 the equilibrium being disturbed. If we 

 suppose a point at which any force acts 

 to be slightly displaced, and from the 

 new position of the point a perpendicular 

 to be dropped upon the direction of the 

 force, then the line intercepted between 

 the foot of this perpendicular and the 

 original position of the point is called the 

 Virtual Velocity of the force. 



1 . Thus, let O be the point at which 

 the force P acts, and suppose it to be 

 slightly displaced, so as to be brought 

 into the position O'; from O' draw the 

 perpendicular O'N on OP, then ON is 

 the Virtual Velocity of P. 



O N 



i^ 



2. The Principle of Virtual Velocities 

 is thus stated : When a system of bodies 

 is in equilibrium under the action of any 

 forces, then if the system be very slightly 

 displaced, the sum of each force multi- 

 plied by its virtual velocity will equal 

 zero. 



VIS. Force ; a term employed by the 

 older writers on mechanics to denote 

 power or force of any kind. Thus we 

 have vis mortua and vis viva ; the former 

 relating to the pressure, the latter to the 

 force of a body in motion, estimated by 

 the distance to which the body goes ; vis 

 matrix, vis acceleratrix, vis inertice, &c. 



VIS INERTIA. Inertness, or the 

 principle of inactivity, by which a body 

 perseveres in the same state of rest or 

 motion, in a straight line, unless obliged 

 to change it by a foreign force. 



VISCOUS FERMENTATION. At a 

 temperature between 86° and 104°, the 

 saccharine juices of plants containing 

 albumen or other azotised matter, un- 

 dergo a species of fermentation, which is 

 different from the vinous, combustible 

 gases being evolved with carbonic acid, 

 and a gummy matter formed, having the 



