LABORATORY. 



the lop of the ash-pit. G H I is half of 

 the mouth of the furnace, which receives 

 a still, or a sand-pot, for distillation, with 

 a retort. This is a little nearer to the 

 front, K, of the top, than the grate-hole 

 is to the front, A, of the bottom, so that 

 the luting 1 is thicker below than above. 

 Near me back, M, of the furnace is a 

 smaller Hole P, for the vent. The luting 

 at Q and It is so formed that the cavity of 

 the furnace does not greatly differ from 

 a cylinder, except in so far as the vent, 

 PO, does not communicate with it abrupt- 

 ly, but is gradually curved downwards, 

 as represented in the figure, making the 

 middle of the cavity more roomy back- 

 wards, by which means it contains a 

 greater quantity of fuel. S is the section 

 of the luting, which forms a sort of an 

 arch, or bridge, contracting the entry of 

 the vent. An iron pipe is set on at P to 

 increase the draught of the chimney. 

 The fuel is put into the furnace by the 

 aperture P, and the sloping form of the 

 cavity causes it to distribute itself pretty 

 uniformly. 



When the furnace is used for smelting, 

 the crucible is set on a pedestal standing 

 on the grate, and the fuel is placed round 

 it with great case, the mouth of the fur- 

 nace being open. This is then shut up 

 by a stopper made on purpose, or by a 

 fiat fire-tile simply laid on it. 



When we would distil with a naked fire, 

 the retort has its bulb resting on a ring 

 which hangs on the mouth of the furnace 

 by three hooks, and the neck of the re- 

 tort lies over the front of the furnace. 

 The space round the retort, at the mouth 

 of the furnace, is closed, as much as is 

 necessary, by two or three pieces of tile, 

 shaped so as nearly to fie the bulb of the 

 retort when they are laid on the mouth of 

 the furnace. A quantity of light ashes 

 are now to be laid on these tiles, and 

 heaped up so as to cover the bulb and 

 part of the neck of the retort. Dr. Black 

 found that this produced a very gradual 

 diminution of the heat, as it recedes from 

 the fuel, and is less liable to crack the 

 retort, by inequality of heat, than any 

 other contrivance. Scarcely any proces's 

 occurs which thisfurnace does not answer 

 with great ease. 



In using the furnaces most convenient 

 for experimental chemistry, (namely, 

 those made of plate iron) it is necessary 

 that the iron be defended from the heat 

 by lining or lute, as we call it, on the in- 

 side ; and such lutes are necessary in 

 other occasions in chemistry ; as when we 

 have occasion to close the joining of the 

 vessels with one another, or to give a 



coating to retorts, or even to crucibles, 

 which is sometimes done. The materials 

 employed for these purposes have their 

 general denomination from clay, of which 

 some of the most useful are partly com- 

 posed, though there are some that do not 

 contain any of it. They may be divided 

 into such as contain animal or vegetable 

 matter, of the glutinous or adhesive 

 kind, and such as are composed only 

 of earthy substances. The first are 

 used for closing the joining of vessels, 

 when the heat we mean to apply is not 

 to be strong, nor the vapours to be pro- 

 duced corrosive. The second serve for 

 the lining of furnaces, or for closing the 

 joinings of vessels, in operations in which 

 the vapours are very corrosive, or in 

 which a strong heat must be employed, 

 which would scorch, or burn and destroy, 

 any animal or vegetable glutinous matter. 



The joinings of vessels with one another, 

 which we have the most frequent occa- 

 sion to close up by means of lutes, are 

 those of retorts with receivers. And we 

 may remark, in the first place, with re- 

 gard to these, that there are not many 

 operations in which it is necessary to 

 make the joining perfectly close, except 

 when the receiver is provided with an 

 air-pipe. On the contrary, it is dangerous, 

 on account of the air which must be al- 

 lowed to escape in some manner. There- 

 fore we are not anxious to contrive the 

 most close and compact. They are suf- 

 ficient and better if they be moderately 

 so, and in some cases, when we think the 

 lute too close, we even obviate it by a pin- 

 hole. The animal and vegetable lutes, 

 employed in this way, are glue and chalk 

 mixed in thin paste, and spread on slips of 

 paper ; or gum arabic and chalk, used in 

 the same manner; or flour and water ; or 

 a bladder ; or linseed meal ; or fat lute. 

 M. Lavoisier recommends, for joinings 

 which we desire to be air-tight, but which 

 are not to be exposed to heat, the follow- 

 ing: to sixteen ounces of bees-wax add 

 one and a half or two of turpentine, and 

 keep it for use. When used, soften and 

 make it tough, by warming and working 

 between the fingers ; then put it on the 

 joint in little rolls, and make it close; and, 

 lastly, cover it with slips of wet bladder 

 laced with pack-thread. But, if the joint 

 is liable to be warmed, or heated, during 

 the operation, you must take fat lute. 

 This is made of raw pipe-clay and linseed 

 oil, beaten together very hard, to the 

 consistence of a stiff adhesive paste. 



Of the second kind of lutes, called the 

 fire-lutes, a great variety have been pro- 

 posed, and some of them compositions of 



