G A 7 ,

G A Z

( 533 )

filled with water, after which the oxygen is letlip by opening the cocks f & g as already de-Ccribed. Then remove the ftop-cock cork andimmediately fubftitute another cork previouflyfitted to the jar, and having the lighted iron wirefufpended from it. The moment it is in thej.ar of oxygen, the iron takes fire and burnswith the phenomena defcribed under that metal.

It may be obferved that as the only intentionof the flop-cock in the cork is to enable the jarto be filled with water, any other method ofdoing this will anfwer as well.

GUN - BARREL APPARATUS. Thisterm is given by fome to the following.

HYDROGEN, Apparatus for procuring.

Under the article Hydrogen we have men-tioned that it is obtained in abundance by pallingthe fleam of water over red-hot iron, andan ingenious apparatus for the purpofe was in-vented by Dr. Prieflley, which has been finceapplied to a number of other experiments of re-fearch, where the objedl has been to fend anykind of vapour through a tube containing anyfubftance heated red-hot. The apparatus isfimply the following : PI. II. Fig. 41. a is a re-tort, the beak of which is luted into one end ofthe tube b b, whieh lies acrofs a portable fur-nace in fuch a manner that the middle partmay be heated red-hot, whilft each extremity iscomparatively cool, c is a bent tube luted tothe other end of b, and which may be diredtedunder an inverted jar in a pneumatic trough,or any other veiTel intended for the reception ofgaffes. r lhe middle of the tube b , which liesover the furnace, is filled with iron wire orturnings clofely rammed. When this part ofthe tube is fully red-hot the water in the retorta is made to boil, by putting a lamp beneath it,and the fleam palling over the heated iron isdecompofed, and the hydrogen produced bythis method is given out copioufly at the benttube c. In this experiment the tube b is gene-rally a plain iron pipe, formed out of a gun-barrel, by filing off the breech. In fome cafesthe tube may be of earth, but it Ibould be re-membered that even the clofeft pottery is forne-what porous at a red-heat, fo that it flioukl beinclofed in another tube of green glafs or metal.

A gun-barrel tube for the fame purpofe isfeen at g, Fig. 77. p], VIII. traverfing a Black’sfurnace, and is mentioned under the articlefurnace. Where the fubftance to be heated inthe tube is of a kind that will not keep itsplace, as clofely rammed iron wire will, thecharge may be confined by a pellet of wet claycat each end, which as it dries Ihrinks and cracks 7

fo as to allow the paflage of aqueous vapourand gas through it, but confines folid or pul-verulent bodies.

HYDROMETER. See Specific Gravity.JAR.

Glafs jars of a cylindrical fhape or nearly fo,are indifpenfably neceflary in the laboratory,both for experiments on gaffes, and for holdingliquors in all the common operations of filtering,precipitation, mixture, &c. For the gaffes, thejars are ufually plain cylinders, Fig. 16. PI. I.of various length and capacity, which ftandfteadily enough when inverted over water, andthe margin of the open end is generally ground-fmooth, that when any flat plate, or a circularbit of card, is preffed upon it, the water maynot drop out when the jar is inverted. For ex-periments with mercury, the jars fliould beffnaller and thicker, and to ftand very fteadily,jars are fometimes wider at bottom, as inFig. 18. The molt convenient jars for mix-tures and precipitation are fuch as Fig. 19, oras thole which are reprefented {landing underthe four funnels in Fig. 65. PI. VI. tliefebeing broad at bottom, any precipitate can bemore thoroughly edulcorated than in any veil'dof the wine-glafs fhape. The thick jar for ex-plofions with the electric fpark, fuch as in Fig.67, has been already defcribed under the articleDetonating Jar.

LABORATORY.

A convenient well-furnifhed laboratory is, ofcourfe, a principal objedl with the chennft,though it may be fome fatisfadlion for the,lover of this attra&ive fcience, whofe means ofindulging it are very limited, to know that mofcof the admirable rcfearches of Scheele , Prieflley,Crawford, and Berthollet , were carried on with-fuch fimple and cheap apparatus as moil per-fons can command. Nevcrthelefs it is of ex-treme advantage to have a well-furnifhed labo-ratory ; accurate (and therefore expenfivc)initruments are aimoft indifpenfible in manyexperiments of refearch ; and if we 1/avc Am-plified many of the long and embarrafling pro-cefles of the older chemifls, we have, on the.other hand, added molt abundantly to the field.-of chemical enquiry by the knowledge of gaf-leous bodies, and equally to the number ofinftruments now neceflary to carry on chemicalrefearches.

It may not be unacceptable to thofe who areentering upon chemical ftudies, to give a fhortlift of thofe articles of furniture which may beonfidered as aimoft indifpenfible to carry on aeneral courfe of experiments, omitting how~