Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/292

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With twelve times the quantity of rain water; put pohfhed plates of copper into this liquor, and the filver will be precipi- tated and the copper diffolved : then put this folution ot cop- per into another glafs, and add to it pohfhed plates of iron ; the copper will be precipitated and cafe over the iron ; finally the copper falls to the bottom, and the iron diffolve?. Pour this folution of iron into a frcfh glafs, and drop upon it od of tartar, per dcliqii'mm ; the diliblved iron immediately tails to the bottom, and the alkali "unites with the acid, and rege- nerates true nitre, after fo many changes. Thus does this fait travel from one body to another almoft un- altered, tho' it is more attracted by one than another,ti!l at length it refts in that which in this refpe& is the ftrongeft, and is only thence expelled when oil of vitriol is poured upon the nitre thus regenerated. On thefe two principles precipitation depends, and is the true, and often abftrufe caufe of numberlefs won- derful operations, both in art and nature. Take agram of white or red precipitate rub it upon a polifhed and heated copper-plate, and wherever the matter has paffed the copper Will immediately look like filver ■ for the copper attraas the acid of the nitre from the calx of the mercury, . and thus pre- fently makes an amalgam upon the furface of the copper, and then acquires a filver colour.

6. Alkalies often precipitate things diffolved by acids. This happens frequently, but not always, nor in perfection : alkali precipitates copper diffolved by an acid, but the copper is after- wards diffolved by a fait made of the two.

7. Acids generally precipitate things diffolved by alkalies; but in this cafe alio there are fome procefies which fliew us exceptions.

8. Sharp falts, without being changed, and lying perfeftly concealed, have ffrange and unexpected effects by means of precipitation. If an ounce of luna cornea, which is perfectly fcentlefs, infipid, and unaftive, and affords no fign of acri- mony in the fire, be ground, and united in a ftrong heat in a glafs retort with half an ounce of inodorous and perfectly in- fipid regulus of antimony, there inftantly arifes an extremely ftrong poifon, or an exceedingly corrofivc butter of antimony, •the exhalation of which proves mortal. We fee in this one inftance how dangerous the art of mixing is, and with what care we ought to go about the compounding of bodies. Boerb. Chem. P. II. p. 338.

PRECIPITATE (Cycl.)~ The different abforbent falts which a/e commonly ufed for the precipitations of metal-', from their folutions in acid menftrua have no effecT: upon thofe metals as to the colour of the precipitate ; but when the folution is clear and limpid, and the matter left on the evaporation of it would have been white, the precipitate in this cafe, by whatever fait it is made, will be white alfo. And when that folution has any particular colour, whether it be that of the metal alone, as in folutions of gold ; or a colour thai! is the rcfult both of the metal and menffruum, as is the cafe in the folutions of copper and iron, the precipitate obtained from the folution will always be of the colour the folution was of, whatever was the fait employed to make it. Tho' this is, however, the cafe in regard to the metals in general, yet it is to be obferved that mercury diffolved in fpirit of nitre, or reduced to the ftate of corrofive fublimate., and afterwards diffolved in water, affords phenomena perfectly different from all thefe : for tho' it gives no colour to the menffruum in which it is diffolved, but lets it remain limpid ; and if the folution be evaporated to a dri- nefs, leaves only a white refiduum, in the manner of the fo- lutions of filver, tin, or lead ; yet inffead of giving a white precipitate, in the manner of thofe metals, it affords a differ- ently coloured one, through a great many varieties, according to the nature of the fait ufed to make it. As mercury is fo- Iuble in more acids than one, and as the experiments made on mena from the fame experiments made on other folutions, Mr. Lemery, who has employed himfelf very nicely on this fub- ject, chofe the folution in fpirit of nitre as the bafis of a long feries of experiments.
 * ne of the folutions of this metal often have different pheno-

As this folution was. a mixture of two bodies, it was firff ne- ceffary to confider them feparately, as to their colours. In this examination it is to be obferved,

T. That fpirit of nitre, when fir ft made, and while it yet con- tains a great quantity of fiery particles, is of a red colour ; and that afterwards, as thefe gradually evaporate and fly off", the fpirit by degrees lofes that colour.

2. That crude mercury, when expofed a long time to the action of the fire, becomes of a red colour.

3. That when a fimple folution of fpirit of nitre is evaporated, and the matter which remains after the evaporation is calcined, it changes from its white colour, which it had at firff, to red, after gaffing through all the {hades of yellow. Now fince, in the change of its ftate from the white to the red colour, the mercury lofes a great part of the acid particles which it had carried away from the folution, we might at firft be apt to conclude, that its change of colour proceeded from this; but on the contrary, experience proves, that of whatever quantity of its acids mercury is. diverted after folution, provided that no frefh particles are admitted in their room, it always con- tinues white fo long as it contains acid enough to prcferve it in this form, or keep it in the condition of a precipitate.

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As we know that fire is itfelf a fluid of a peculiar nature, and which has a power like other fluids of infinuating itfelf into the pores of the other bodies, and there, like them, prefac- ing its effential properties ; and as we alfo know, that the mixture of the particles of fire, either fingly with mercury, or with fpirit of nitre, gives to either a red colour ; it is very natural to conclude, that the quickfilver, when it has been penetrated by the acid of fpirit of nitre, and afterwards acted upon by calcination, is not changed from white to yellow, and from yellow through all its changes to red, by any other means than by the particles of fire which introduce themielves into the precipitate as tbey drive the acid particles out : and this is evinced the more clearly by this experiment, that if frefh acid be added to this precipitate, now become red, in fuch proportion as to diveff it again of the fiery particles, and lodge its own in .their places, then the whole lofes its red colour, and cither becomes colourlcfs, or elfe white as it was before. The common white precipitate acted upon by a flow fire, thus lofes its white colour, and advancing through all the de- grees of yellow, becomes at laff red, as in the former inftance ; but if, inffead of this flow heat, there is a ftronger fire em- ployed, and that continued fo long as to ra'de this precipitate in a matrafs into the form of a fublimate, in this cafe it pre- ferves its white colour, notwithftanding the violence of the fire andthelofs of itsacid particles,whichinthis procefs cannot but be very confiderable ; nay, and tho' the fublimation he repeated feveral times, the whitenefs of the matter will not be altered hy it. From this obfervation it may be feen, that when nothing is added to the matter in the place of the acid particles which arc driven off", it fufters no change of colour by their lofs ; and that in cafes where larger fires are ufed, and confequently the fiery particles are driven through the body of the precipi- tate, there is no change of colour made in it by them ; but that in order to make this change of colour, the fire muff be flow, and the particles muff be driven in with a force that is not fufficient to drive them out again, and confequently muft: remain there. It is evident, that the deprivation of the acids alone, does not change the white precipitate red ; becaufe in this cafe, where there is no fuch change of colour, there is, however, a manifeft deprivation of a great part of thofe acids ; fince there are always many globules of revived mercury found among the maffes of the fublimate. Experiment proves, that mercury becomes more or lefs able to refift the force of fire, as it is more or lefs highly impregnat- ed with acids : and hence the fame degree of fire applied to two quantities of mercury, differently charged with acids, fhall have different effects, and fhall give a red colour to that which. is fo fated with acids that it will not beraifed in fublimation ; and the other, which is lefs charged with them, fhall be fub- limed and left white. The white precipitate made in the com- mon way, is at the utmoft only able to bear a much fmaller degree of fire than that neceffary to make the common red precipitate; and crude mercury itfelf can only bear a much weaker than either, as is (een in the making the common cal- cined mercury, improperly called precipitate per fe. There is this, however, very remarkable in the making this calcination, that after the mercury has been calcined fome time with the neceffary gentle heat, it becomes able to bear a much ftronger degree than it could at firft. This is exactly the contrary of what is found in making the common red precipitate ; in doing which the fire muft necefiarily be gradua'ly diminifh- ed, for the fame degree of fire which it at firft bore, nay, which was neceffary to it, if continued, would certainly fub- lime and evaporate it. The explanation of this is, that in each cafe the mercury becomes able to refift the force of the fire in proportion as it receives particles which increafe its weight. This both fire and the acid are capable of doing to it ; but the acid does it in fo greatly fuperior a decree, that while in the one cafe crude mercury, receiving particles of fire into its body, becomes by their means capable of refifting a ftronger heat ; in the other, thefe particles of fire being only received in the place of acid ones before diflipated, which added much more to its weight than they, it becomes much lefs able to refift a violent heat by, the change, and gradually, as it receives the fiery in the place of the acid particles, re- quires the heat to be decrcafed, otherwife it muft be evapo- rated.

In fine, we fee that crude mercury requires ?. long time to be reduced into a red powder by means of calcination; whereas the fame mercury, when loaded with acids, may be converted into this red powder in a few hours ; the one bearhig a tole- rably ftrong beat, the other only a very weak one. It appears that the mercury loaded with acids yields, therefore, a more eafy accefs to the particles of fire than the crude ; and what proves this, and evidently {hews that the acids ac- celerate the effect of the fire on this body is, that if the com- mon white precipitate he expofed to the fame gentle degree of heat with crude mercury, it becomes red much fooner; and as the difference here is only, that the one is plain mercury, the other mercury penetrated by acids, there can evidently be no reafon for the latter turning red fooner, but its being fo penetrated.

After thus examining the effects of fire on folutions of mer- cury, this author proceeds to examine the effects of thetdiffer- 3 ent