Part 33 (1/2)

In thisit is ias disengaged, because a part of it is absorbed by the water while passing through it; but, when the carbonic acid is absorbed, the azotic gas reitated for a few minutes in caustic alkaline solution, we obtain it pure, and can easily deterht We e of the quantity of carbonic acid by repeating the experi the proportions of charcoal, till we find the exact quantity requisite to deflagrate the whole nitre eht of charcoal een necessary for saturation, and deduce the quantity of oxygen contained in a given weight of nitre

I have used another process, by which the results of this experiment are considerably asses in bell-glasses filled with h to contain jars of from twelve to fifteen pints in capacity, which are not very readily ed when full of mercury, and even require to be filled by a particular lass syphon is introduced, connected with a small air-pump, by means of which the air is exhausted, and the as of the deflagration is made to pass into the jar in the saain repeat, that this species of experireatest possible precautions I have soas proceeded with too great rapidity, jars filled with more than an hundred and fifty pounds of mercury driven off by the force of the explosion, and broken to pieces, while the reat quantities

When the experias is collected under the jar, its quantity in general, and the nature and quantities of the several species of gasses of which the mixture is composed, are accurately ascertained by the methods already pointed out in the second chapter of this part ofthe last hand to the experiration, froed in; and I aht upon the operations belonging to the un-powder

CHAP X

_Of the Instruh Temperatures_

SECT I

_Of Fusion_

We have already seen, that, by aqueous solution, in which the particles of bodies are separated from each other, neither the solvent nor the body held in solution are at all decomposed; so that, whenever the cause of separation ceases, the particles reunite, and the saline substance recovers precisely the same appearance and properties it possessed before solution Real solutions are produced by fire, or by introducing and accureat quantity of caloric between the particles of bodies; and this species of solution in caloric is usually called _fusion_

This operation is commonly performed in vessels called crucibles, which must necessarily be less fusible than the bodies they are intended to contain Hence, in all ages, chemists have been extremely solicitous to procure crucibles of very refractory ree of heat The best are made of very pure clay or of porcelain earth; whereas such as are made of clay mixed with calcareous or silicious earth are very fusible All the crucibles hbourhood of Paris are of this kind, and consequently unfit for most cheood; but the best are es earth, which seereatcrucibles; such, for instance, is the kind used for lass-manufactory of St Gobin

Crucibles areto the operations they are intended to perform Several of the7 8 9 and 10 the one represented at Fig

9 is alh fusionthe nature of the fused body, this operation is frequently e bodies In this way all the metals are extracted from their ores; and, by this process, they are revivified, moulded, and alloyed with each other By this process sand and alkali are colass, and by it likewise pastes, or coloured stones, enamels, &c are formed

The action of violent fire was much more frequently employed by the ancient chereater precision has been employed in philosophical researches, the _humid_ has been preferred to the _dry_ method of process, and fusion is seldom had recourse to until all the other means of analysis have failed

SECT II

_Of Furnaces_

These are instruments of reat nu well or ill constructed, it is of great importance that a laboratory be well provided in this respect A furnace is a kind of hollow cylindrical tower, so 1 ABCD, which s; one in its upper part F, which is the door of the fire-place, and one below, G, leading to the ash-hole

Between these the furnace is divided by a horizontal grate, intended for supporting the fewel, the situation of which is h this be the least complicated of all the chereat nueneral, every substance which does not require a very strong fire, may be melted in crucibles; it will serve for metallic oxydations, for evaporatory vessels, and for sand-baths, as in Pl III Fig 1 and 2 To render it proper for these purposes, several notches, e, as otherwise any pan which e of the air, and prevent the fewel froree of heat, because the quantity of charcoal it is capable of consu is limited by the quantity of air which is allowed to pass through the opening G of the ash-hole Its power , but then the great streaht be hurtful in others; wherefore we must have furnaces of different forms, constructed for different purposes, in our laboratories: There ought especially to be several of the kind now described of different sizes

The reverberatory furnace, Pl XIII Fig 2 is perhaps more necessary

This, like the common furnace, is composed of the ash-hole HIKL, the fire-place KLMN, the laboratory MNOP, and the dome RRSS, with its funnel or chimney TTVV; and to this last several additional tubesto the nature of the different experiments The retort A is placed in the division called the laboratory, and supported by two bars of iron which run across the furnace, and its beak comes out at a round hole in the side of the furnace, one half of which is cut in the piece called the laboratory, and the other in the dome In most of the ready made reverberatory furnaces which are sold by the potters at Paris, the openings both above and below are too small: These do not allow a sufficient voluh; hence, as the quantity of charcoal consu, the quantity of caloric disengaged, is nearly in proportion to the quantity of air which passes through the furnace, these furnaces do not produce a sufficient effect in a great nuht to be two openings GG to the ash-hole; one of these is shut up when only a moderate fire is required; and both are kept open when the strongest power of the furnace is to be exerted The opening of the doer than is usually reat ie size in proportion to the furnace, as a sufficient space ought always to be allowed for the passage of the air between the sides of the furnace and the vessel The retort A in the figure is too small for the size of the furnace, yet I find it more easy to point out the error than to correct it The intention of the doe the flame and heat to surround and strike back or reverberate upon every part of the retort, whence the furnace gets the name of reverberatory Without this circumstance the retort would only be heated in its bottom, the vapours raised from the contained substance would condense in the upper part, and a continual cohabitation would take place without any thing passing over into the receiver, but, by means of the dome, the retort is equally heated in every part, and the vapours being forced out, can only condense in the neck of the retort, or in the recipient

To prevent the botto either heated or coolled too suddenly, it is so upon the cross bars of the furnace Likewise, in many operations, the retorts are coated over with lutes, some of which are intended to preserve them from the too sudden influence of heat or of cold, while others are for sustaining the glass, or forlass one during operations wherein the strength of the fire ht soften it The former is st with it, into a paste or lass or stone retorts The latter is ether, and used in the same manner This dries and hardens by the fire, so as to for the lass retort below should crack or soften But, in experiasses, this lute, being porous, is of no reat many experiments wherein very violent fire is not required, the reverberatory furnaceout the piece called the laboratory, and placing the dome i 3 The furnace represented in Fig 4 is very convenient for fusions; it is composed of the fire-place and ash-hole ABD, without a door, and having a hole E, which receives the ly luted on, and the doht to be rather lower than is represented in the figure

This furnace is not capable of producing a very strong heat, but is sufficient for ordinary operations, and may be readily h these particular furnaces are very convenient, every laboratory ood pair of bellows, or, what isfurnace I shall describe the one I use, with the principles upon which it is constructed

The air circulates in a furnace in consequence of being heated in its passage through the burning coals; it dilates, and, beco air, is forced to rise upwards by the pressure of the lateral columns of air, and is replaced by fresh air from all sides, especially from below This circulation of air even takes place when coals are burnt in a co dish; but we can readily conceive, that, in a furnace open on all sides, theequal, cannot be so great as when it is obliged to pass through a furnace in the shape of a holloer, like most of the chemical furnaces, and consequently, that the combustion must be more rapid in a furnace of this latter construction Suppose, for instance, the furnace ABCDEF open above, and filled with burning coals, the force hich the air passes through the coals will be in proportion to the difference between the specific gravity of two columns equal to AC, the one of cold air without, and the other of heated air within the furnace ThereAB, and the superior levity of this ought likewise to be taken into consideration; but, as this portion is continually coolled and carried off by the external air, it cannot produce any great effect

But, if we add to this furnace a large hollow tube GHAB of the same diameter, which preserves the air which has been heated by the burning coals fro air, the difference of specific gravity which causes the circulation will then be between two coluth of AC, the circulation will have treble force This is upon the supposition that the air in GHCD is as much heated as what is contained in ABCD, which is not strictly the case, because the heat must decrease between AB and GH; but, as the air in GHAB is much warmer than the external air, it follows, that the addition of the tube er quantity reater degree of combustion must take place