DIG

D I S

( 517 )

CUPEL. PI. I. Pig. 28 and 29.

1 his is a folid pafte of bone-afh moulded ina {hort cylindrical or truncated pyramidal form,with a Ihallow circular cavity at the top, aboutthe fize of a fegment of a large bullet. It isufed in the procefs of AJfaying, (which articlefee in Vol 1. p. no.) In making cupels thepowdered bone-afh is moiftened with a verylittle water, and the mafs is made into therequired form by being {truck with confider-able force into a mould of brafs or iron, andafterwards gradually dried. By this means itacquires fufficient hardnefs for the ufe to whichit is put.

DETONATING JAR. PI. VI. Fig. 67.

It is frequently neceffary in the analyfis offome gaffes to take the eleftric fpark on givenquantities in clofe veffels, fuch as mixtures ofoxygen and hydrogen, which lalt forms theeudiometer contrived by Volta, and bears hisname. It confilts of a thick glafs tube quiteopen at bottom, and near the top are twoperforations in which fmall wires are cementedthat approach each other within the jar nearenough to communicate the eletlric fpark,from an adjoining machine, whereby the in-clofed gaffes are fired. In molt cafes wherethis apparatus is ufed it is with mixtures ofgaffes that detonate ftrongly when fired, whichis attended with fome inconveniences. If thejar in which the experiment is fired is clofelyflopped a very fmall quantity of hydrogen andoxygen will burft pretty thick glafs; and if itis entirely open, fo that all the force of theex-plofion may be fpent on the liquid in whichthe lower end is immerfed, a good deal of it(efpecially if it be mercury) is thrown out anddiiperfed to a diflance. Thefe inconveniencesare entirely remedied by the detonating jardeferibed in the Plate (invented by Mr. Pepys )in which the jar is fupported by the heavy ironHand c, which has a ftrong fpring near the footthat draws out in the manner of the fteel-yardwhen the concuffion is given •, and thus muchlarger quantities may be fafely detonated thancan be done by the common apparatus, with-out fpilling any of the mercury. The {hock istaken by wires that pafs through the holesa and b.

DIGESTER.

.A digefter is a thick iron boiler furnifhed■with a ftrong lid perfe£lly tight, that may beferewed down with force fufficient to refill theexpanfive force of the inclofed fleam, wherebythe water or other liquid which it containsmay be heated confiderably higher than the

natural boiling point of water when unconfined.But as the llrength of the veffel diminifhes,and the force of expanfion increafes as the heatis raifed, there is a point at which the veffelwould burll, and a dangerous explofion wouldtake place, if it were not prevented by a fafetyvalve, with which all thefe veffels are furnifhed,and which will open and let part of the con-fined vapour efcape when the expanfion hasreached a certain point. There are fome cafeswhere a digefter proves an ufeful part of che-mical apparatus, but it is not often employed.

DIS FILLING APPARATUS.

Under this article we fhall deferibe the dif-ferent kinds of alembic, the common flill, withits worm-tub, and the other apparatus forfitnple diftillation, or that in which the foieobjedl is to condenfe and collect in a liquidfrom the heated vapour of anv fubftance.When a part of the produdl to be coile£ted iseither a permanently elaflic gas, or a vapourwhich requires to be paffed through water orany other liquid in order to be condenfed, it isneceffary to employ a more complicated appara-tus fimilar to that which was originally inventedby the late Mr. Woulfe, and will be deferibedunder the article Woulfe’s Apparatus.

The fimplefl apparatus for diftidation withmoderate quantities is the Retort and Receiver.The former is made either plain (PI. I. Fig. 5.)or tubulated and fitted with a ground ftopperFig. 2 . It fhould be roomy at the bend, as it isthere where the vapour firfl begins to condenfe.When the plain retort is ufed, the materials tobe diddled, if liquid, fhould be poured in by avery long funnel, fo that none of it may trickledown the neck. The bottom of the retortwhere the heat is applied is blown pretty thin,and being of uniform thicknefs it will alwaysbear the heat of an Argand lamp, or even itmay be flung upon a fmall chain over a com-mon open coal fire when burning clearly, withvery little rifk of breaking, if tolerable cautionbe ufed. The neck of the retort pafles intothat of the receiver, or fometimes with the in-tervention of an Adopter (Fig. 3.) and the join-ings fhould be luted. The receiver fhould beof confiderable capacity compared to the retort,fuch as Fig. 4. to Fig. 2 . to allow a large coolfurface for the condenfing vapour, and it maybe further kept cool by covering it loofely witha cloth frequently wrung out of cold water.The receiver may be either a fimple globe witha neck Fig. 6, or may be drawn out into a ior.gtube at bottom, as in Fig. 4, in which cafethe tube fhould enter a bottle placed beneath,.