VOL 



VUL 



advancing to any distance on tlie ground 

 or on trees. They are divided into the 

 Cypselinae or Swifts, the Hirundinae or 

 Swallows, and the Caprimulginae or Goat- 

 suckers. 



VO'LKONSKOITE. A mineral con- 

 taining oxide of chromium, and found in 

 thin veins and nests in Mount Jess- 

 mictski, in Siberia. 



VOLTA-ELECTROMETER. A new 

 instrument for the exact measurement 

 of electric currents. The current to 

 be measured is made to pass through 

 water acidulated by sulphuric acid, and 

 the gases evolved by its decomposition 

 are collected and measured, thereby 

 giving at once an expression of the quan- 

 tity of electricity which has passed. The 

 principle on which this conclusion is 

 founded is this new law^ " that the decom- 

 posing action of any current of Elec- 

 tricity is constant for a constant quantity 

 of electricity." 



VOLTA-METER. An instrument for 

 measuring the intensity of the electric 

 current. It consists of a cell of decora- 

 position, containing dilute sulphuric 

 acid, and so formed as to admit of the 

 collection and measurement of the 

 evolved gases. The electrolyte which 

 best fulfils all the requirements, is water ; 

 and any arrangement in which a stream 

 of electricity is made to decompose it, if 

 furnished with the means for measuring 

 the quantity of gas thus generated, may 

 be regarded as a volta-meter. 



VOLTAIC CIRCLES. If a plate of 

 zinc and a plate of copper be immersed 

 to a certain depth in a vessel containing 

 dilute sulphuric acid, and connected 

 together by direct contact, a current of 

 positive electricity passes uninterruptedly 

 from the zinc through the liquid to the 

 copper, and from the copper to the zinc, 

 in the direction of the arrows. Such 

 an arrangement is termed a simple 

 voltaic circle. "When it is required to 

 produce more decided effects, a series 

 of these plates is combined, form- 

 ing what is called a compound voltaic 

 circle. 



VOLTAIC ELECTRICITY. The phe- 

 nometaa of electricity, observed by Gal- 

 vani, and hence termed Galvanism and 

 animal electricity, were attributed by 

 Volta to the disturbance of the electric 

 equilibrium by the contact of two dissi- 

 milar metals. The identity of the com- 

 . mon and the galvanic electricities was 

 proved by the discovery of the voltaic 

 pile, and tlie science has from that period 

 371 



been termed by some writers, Voltaism 

 or Voltaic Electricity. 



VO'LTATYPE. A term synonymous 

 with electrotype, applied to a new art of 

 plating performed by electro-chemical 

 action. 



VOLTZITE. Oxysulphuret of zinc, 

 occurring in the form of small hemi- 

 spheres, divisible into thin layers. It 

 occurs at Roziers in France, and, as is 

 said, in Cornwall. 



VOLUBLE [voluhilis, from volvo, to 

 twist). Twisting; as applied to stems 

 which twist or twine round other bodies, 

 that of the hop to the right, that of the 

 bindweed to the left. 



VOLUME {volumen, from volvo, to 

 roll). The apparent space which a body 

 occupies is called its volume; the effec- 

 tive space which the same body occupies 

 or its real bulk of matter, is its mass ; 

 the relation of the mass to the volume 

 (or the quotient of the one by the other) 

 is its density ; and the empty spaces, or 

 voids, vrhich render the volume larger 

 than the mass, are its pores. 



Definite Volumes. The union of gases 

 is always effected in simple proportions 

 of their volumes : a volume of one gas 

 combines with an equal volume, or twice 

 or three times the volume, of another 

 gas, and in no intermediate proportion ; 

 this is called the law of definite volumes. 



VOLU'TID^. The Volutes; a fa- 

 mily of testaceous, carnivorous gastero- 

 pods, named from the typical genus va- 

 luta; the column of the shell is regu- 

 larly plaited, and the mantle much de- 

 veloped in the typical species. 



VOLUTION {volutus, rolled). A term 

 synonymous in Malacology with whorl. 



VOLVA {volo, to roll). The wrapper, 

 or involucrum-like base of the stipes 

 of agaric. It originally was a bag en- 

 veloping the whole plant, and was left at 

 the foot of the stipes, when the plant 

 elongated and burst through it. 



VORTICES, HYPOTHESIS OF {vor- 

 tex, a whirlpool). The primary hypo- 

 thesis of the natural philosophy of Des- 

 cartes was, that the universe is a, plenum, 

 that is, without any vacuum or unoccu- 

 pied space ; and that the atoms of matter 

 moved in numerous vortices, which car- 

 ried the heavenly bodies around their 

 several centres of motion ; such as the 

 planets about the sun, and, perhaps, 

 similar planets around the fixed stars. 



VULPECULA ET ANSER. The Fox 

 and the Goose ; a modern northern con- 

 stellation, consisting of thirty-seven 



