VAL 



VAR 



attached to the centre of the plate per- 

 pendicularly to its surface, and moves 

 freely up and down in a perforation in 

 each of two bars fixed diametrically 

 across, and near the top and bottom of 

 the orifice. 



4. The spherical valve is a modification 

 of the conical valve : the seat or nozzle 

 represents a portion of a sphere; the 

 valve itself may also be a portion of a 

 sphere, or an entire sphere. It has been 

 recommended as a safety-valve, to pre- 

 vent the danger of adhering in the boilers 

 of steam vessels. It is also called a cup- 

 valve. 



5. The throttle-valve has, instead of 

 the reciprocating motion of the valves 

 already noticed, a rotary motion. Its 

 action is that of the flood-gate of a mill ; 

 its purpose is to regulate the power of a 

 steam-engine, by increasing or diminish- 

 ing the area of the steam-tube of a 

 steam-engine, and thus to increase or 

 diminish the amount of steam supplied 

 to the cylinder. The name of this valve 

 suggests its use: the steam- tube is throt- 

 tled by it. 



6. The sliding valve is one which does 

 not rise from, and fall into, a seat or 

 nozzle, but slides on and ofl" its aperture ; 

 hence, it is commonly called a slide. In 

 modern steam-engines, the steam pas- 

 sages are opened and closed by the action 



j of a single slide, which performs the office 



of four valves. To this head belong the 

 . slides of Seaward, Murray, Murdock, &c. 



[ The last of these, from its semi-cylin- 

 • drical form, is commonly called the 

 D-slide. 



7. The single cock is a kind of valve 

 consisting of a plug of a nearly cylindri- 

 cal shape, inserted into a hole of cor- 

 responding form and dimensions in a 



I tube ; the plug is perforated by a large 



hole, and turned by a handle outside the 

 tube. The four-passaged cock is a con- 

 trivance, founded on the principle of the 

 common cock, for putting four passages 

 into communication with each other, 

 alternately by pairs ; it is used in steam- 

 engines for establishing communications 

 between the boiler, two cylinders, and the 

 external air. Instead of this apparatus, 

 two doubled- passaged cocks may be em- 

 ployed : the four-passaged becomes a 

 double-passaged cock by obliteration of 

 one of the passages. 



VALVES (in Botany). 1. A term ap- 

 plied to the parts into which certain 

 fruits separate, exhibiting the various 

 ' forms of valvular dehiscence — the septi- 

 357 



cidal, the loculicidal, and the septlfragal j 

 the axis of the fruit from which the 

 valves separate in cases where a distinct 

 axis exists, is called the columella. 2. The 

 term valve has been also applied collec- 

 tively to the three classes of bracts of 

 which the flower of grasses is composed. 

 3. It also denotes the opening in the 

 cells of anthers, which occurs when the 

 pollen is about to be discharged. 



VALVES (in Zoology). The term 

 valves denotes the two ordinary or prin- 

 cipal pieces which form the covering of 

 the acephalous testacea, or bivalves^ 



VANA'DIUM {Vanadis, a Scandina- 

 vian deity). A rare metal discovered by 

 Sefstroem in 1830, in the iron prepared 

 from the iron ore of Taberg, in Sweden, 

 and procured afterwards in larger quan- 

 tity from the slag of that ore. It was 

 subsequently discovered in a new mine- 

 ral, the vanadiate of lead. It occurs in 

 the state of vanadic acid. 



VANISHING QUANTITY. In Ma- 

 thematics, a quantity is said to vanish^ 

 or to become evanescent, when its arith- 

 metical value is nothing, or denoted by 0. 

 See Fraction. 



VAPORIZATION. The conversion 

 of a liquid or of a solid budy into vapour 

 by the application of heat. It comprises 

 the phenomena of evaporation and of 

 ebullition. When solid bodies are vapor- 

 ized and subsequently condensed, the 

 operation is termed sublimation, and it 

 is employed for the purification of cer- 

 tain substances, and for other pur- 

 poses. 



VAPOUR. A light, expansible, and 

 generally invisible gas, resembling air 

 completely in its mechanical propertieSr 

 while it exists, but subject to be con- 

 densed into the liquid or the solid form 

 by cold. Vaporizable bodies are termed vo- 

 latile ; while those which resist the heat 

 of the furnace without undergoing vapor- 

 ization, are said to ^ie fixed in the fire. 



Vapour, atmospheric. The watery 

 vapour existing in the atmosphere from 

 contact with the surface of the sea, of 

 lakes, of rivers, and of humid soil. Its 

 quantity is limited by temperature, and 

 when this is reduced, the watery vapour 

 is condensed, and becomes visible in the 

 form of dew, clouds, rain, &c. Vapour, 

 when visible, is termed vesicular, from its 

 appearing, in that state, in the form of 

 minute vesicles. 



VARA'NIDiE (TarawMJ, the monitor). 

 The Monitors ; a family of Lizards, 

 known as the Platynote or Broad-backed 



