BAT

BL O

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K. L.) which confifts of an iron pot funkinto a furnace, and filled with fand. It takes alonger time to heat than the water does, and theheat is much lefs uniform through the fand,being confiderably greater at the bottom of thefand than at the top. The utmoll heat ufuallygiven to the fand-bath is that of low rednefs,and it has the advantage of keeping very hot for•a long time, without any additional fuel.

In a few inftances ftrong brine, "or mercury,are ufed as balnea. The former when boilingis confiderably hotter than fimple water, fothat water immerfed in boiling brine may bebrought fully to the boiling point. Mercurywill receive a much greater heat, and itperhaps might be more frequently ufed withadvantage than it is at prefent, for heat pe-netrates through it with much more rapiditythan through water, and every veffel ufed inchemiftry with their materials, will float uponits furface. The mercury fhould be in an ironpan, and fhould be kept much below its boilingor evaporating point.

The terms water and mercury bath are oftenufed, though not quite corredtly, for the Pneu-matic Troughs of thefe fluids ufed in ope-rations on gaffes (which fee).

BELL GLASS. BELL RECEIVER. PI. I.Fig. 17 -IV. 52-

This is a very ufeful form to be given to largeglafs veflels in a variety of experiments on gaffes,particularly v/here any vacuum is produced, asthe vaulted form of the veffel refills the greatforce of atmofpherical preffitre. It is thereforeemployed- in experiments with the air-pump.BLAST FURNACE . See Furnace.BLOW-PIPE. PI. VII.

When a fmall jet. of air is directed laterallyinto the middle of the flame of a lamp or candle,a portion of the flame is diverted in the dire£tionof the jet, and forms a long flender cone offlame, the heat of which increafes towards theend of the cone,, and at the very point its in-tenfity is extreme. This affords a molt conve-nient way of, heating in a very high degree witha moll cheap and fimple apparatus, and almoftinftantaneoufly, any fubftance minute enoughto be involved by the flame, and of performinga vaft number of experiments by which thenature of fubltances may be difeovered, thoughthe actual quantities employed are generally toominute to enable the chemift to afeertain rela-tive proportions with accuracy.

The inftrument by which the jet of air isproduced, is called the blow-pipe, of whichthere are feveral kinds. The moft common is

that ufed by the mouth, and need not be morethan a tube of brafs or any other, material, bent,near the end the furtheft from the mouth, anddrawn out fufliciently fine to keep up a conftantrapid ftream of air by the fupply that the mouthis able to furnifh.

But as in the courfe of blowing, the moiftureof the breath is apt to condenfe in a troublefomequantity, an improvement has been adopted ofmaking a bulb near the fmall end of the pipe,which, to render it more portable, is dividedthrough the middle, and the two parts ferewtogether when ufed. Fig. 73 a b is a commonbrafs blow-pipe of this kind, and inllead of thepipe itfelf terminating in a fufliciently fmallopening, a feparate jet-pipe (Fig. 72) is flippedon, of which there are two or three differentfizes in the bore, according as a larger and moremoderate, or a fmaller and more intenfe flamesis required.

Fig. 71 is a blow-pipe of the fame kind, butfrill more convenient. It contains, like the for-mer, a circular enlargement to condenfe themoifture, but the fmaller end d is moveable inany direction round the center c, whereby anydegree of obliquity may be more convenientlygiven to the flame. The jet pipe, Fig. 72 is ufed.as.in the lalt, and an ivory mouth piece, b, flipson the larger end, being pleafanter to the lips.

The belt kind of flame for blowing throughwith the common blow-pipe is a wax or tallowcandle with a very large wick, which fhould bekept fnuffed moderately low, and the wickturned a little afide from the pipe'. A fpiritlamp is fometimes ufed, which makes a per-feftly clear flame without fmoke, but weakwhen ufed in this way. There is a kind ofknack in blowing with the mouth, which is noteafily deferibed, and requires a little practice tobe performed with eafe. As the flame mudoften be kept up for feveral minutes, the a£t ofrefpiration muft be carried on through thenoftrils without interruption, and the ftrefs ofblowing mult be performed merely by com-preflion of the cheeks upon the air in the mouth.

The fubftance to be heated is placed eitheron a piece of charcoal or a metallic fupport.When the former is ufed, a large clofe well-burnt piece of charcoal muft be chofen, a fmallfliallow hole fcooped out with a knife, and thefubftance laid upon it. The charcoal itfelfkindles, all round the Bole, and the hole is thusgradually enlarged; and the heat too is keptup round the fubftance much more uniformlythan when a metal fupport is ufed. At thefame time however the chemical died! produced