A L K
A L K
( 34 )
known substance, and this renders it a mostvaluable reagent for a variety of interestingchemical decompositions.
Before we proceed with the other propertiesof potassium we shall describe another methodof procuring this substance without the assist-ance of any galvanic apparatus, and in muchlarger quantity. This method was found outby Messrs. Gay Lussac and Theuard, soonafter Mr. Davy’s original discovery of potassium,and is described by them with much minute-ness. h It consists in heating iron filings togreat intensity in an iron gun-barrel, and thenpassing melted potash through them. Thepotash is decomposed, a large quantity of hy-drogen gas is given out, and the potassium iscondensed at the further extremity of the ap-paratus. As this, though not very compli-cated, requires a good many precautions, weshall give the original process with minute-ness.
The materials. Iron turnings are preferableto iron wire or nails, as they present a consider-able surface and yet allow the alkali to passfreely through them. They should be brokendown in a mortar, and any adhering oxyd siftedaway. The choice of the alkali is not a matterof indifference. The common potash or pearl-ash always contains a small portion of soda,and hence the caustic potash prepared from itis mixed with a little soda, which causes analloy of a small quantity of sodium with thepotassium obtained. Therefore in delicate ex-periments of research the potash should beprepared from pure crystallized nitre and creamof tartar ignited together, or from calcinedcream of taitar alone, which gives the purestpotash known. The alkali should be madecaustic with lime in the way mentioned underthe article potash, (Ch. Diet.) and indeed theextraction of the alkali by alcohol is hardlyrequired, but it should be kept at a red heat forsome time just before being used, as the pre-sence of much moisture defeats the object ofdecomposition of the alkali.
The apparatus consists of a gun-barrel A.B.C. (see plate, fig. i and 2.) which should bewell cleaned within and then bent in twodirections by being heated red-hot at the partsrequired. The middle of the barrel B.C. whichis enclosed in the furnace, must be well coveredwith a very refractory lute, as it is to undergoa very intense heat for a considerable time,which would soon oxydate and destroy thebarrel without such protection. The best lute
is a mixture of pipe clay (or Windsor loam)beat up with a little water, and incorporatedwith as much sand as possible so as to make itdifficult to apply. The only part to be lutedis that which lies across the furnace, and tothis the lute is to be applied in three or foursuccessive thin coatings, and dried with greatcare, first for a few days in the shade, andthen with a gradual heat, filling up any littlecracks in it by some fresh lute. The gun-barrel is then filled with the iron turnings, asfar as from B. to C. and is then firmly fixed inthe furnace, and afterwards the fragments ofrecently heated potash are put in, withoutbeing much pressed, so as to reach not quite toB. and up to a few inches below A. Thisrequires generally about 3 ounces troy, moreor less. As much gas- always issues throughthe other extremity D, to prevent any mis-chief or loss of potassium when accidentallystopped up, it is of use to adopt a bent glasstube to the upper end of the barrel A. whichdips under mercury M. as seen in the plate, fig. 1.and allows of the reflux of any gas from thebarrel if necessary.
The apparatus being then put up, a fire islit in the furnace, and the bellows applied atfirst very gradually, to prevent the lute fromcracking. The recipient G. H. is formed bytwo copper tubes fitted to each other bygrinding, the uppermost of them receiving theend D. of the gun-barrel. A glass tube F. fitsinto the lower half of the recipient anil dipsinto mercury. The recipient is luted on tothe gun-barrel by the same fire-lute. Whenthe fire is well kindled the bellows are to beused vigorously, and the heat raised to a veryhigh degree at the same time that the ends ofthe barrel that.project beyond the fire abovearid below, are kept cool by wetted cloths.When the fire has been raised to it* utmostfor some time, the wet cloths are removedonly from the upper end of the barrel thatcontains the potash, and instead of it either asmall furnace, or chafing dish or hangingtrough of iron, E. filled with hot charcoal,is to be applied so as to cover the lowerpart of that portion of the barrel that con-tains the potash, and to melt the alkali, whichrequires a moderate red heat. The meltedalkali then falls down into the lower partof the barrel, which is intensely heated, andpasses in the form of vapour through the ironturnings. At first the water which the alkalicontains in abundance (thoughpreviously fused)
k Rechercfies Rhysico-Clumiques, tom. 1.