—Rectifying Retorts.

942 SULPHURIC ACID.

able to retain. Nitrous compounds also are found,together with traces of hydrochloric acid. Traces ofselenium have also been detected. Arsenic is a com-mon impurity, derived usually from the iron pyrites.Traces of arsenic, too, have been detected even invitriol made from commercial sulphur. For thegreat majority of purposes these impurities are notfound inconvenient in the arts; while, on the otherhand, for some special uses a process of purificationmust be had recourse to.

Sulphuric Acid —Rectified Oil of Vitriol—English Oil of Vitriol .—It has been stated previously thatthe concentration cannot be safely conducted in leadbeyond the density of T750. The further concen-tration, then, must be performed in vessels made ofa material which will withstand the corrosive actionof the boiling acid, and which is comparativelyinfusible. Glass at once suggests itself or somevariety of earthenware, such as porcelain, or, amongstthe metals, platinum or gold. Of the former twoglass offers most advantages; and of the two lattermaterials platinum is preferable, inasmuch as itresists acids quite as well, and is much cheaperthan gold. Glass has been employed in the con-centration of vitriol from very early times, perhapsfrom the period when sulphuric acid began to beunderstood, and its manufacture attempted on avery small scale, doubtless before the introductionof leaden pans. Platinum, on the other hand, is acomparatively late introduction. The glass vesselsemployed in the last concentration of sulphuric acid,or, as they are technically known, retorts, resemblevery much the bottles or carboys used to transportvitriol from place to place. Fig. 12 represents the

Fiff. 12.

Arr.rUf

whole arrangement when at work. The retortsvary in size; those in use at the present day arecapable of holding more than 20 gallons of liquid.

The mode of working can be gathered fromthe drawing, where a represents the fireplace; theflame from the fire plays round a cast-iron pot,finally making its exit into the flue. On the bot-tom of the iron pot just mentioned a quantity ofdry sand is laid; and on this the retort, B, is placed,the sand serving as a steady support for it. Theretort is not, as is sometimes supposed, surroundedwith sand; for, if this were done, it would run somerisk of being broken, besides materially retardingthe cooling when the rectification was over.

The retort being placed, as in the drawing, it isfilled with warm brown vitriol, and when this isdone the fire is lighted; the arm, e, is put into itsplace; eventually the acid boils; water, with a littlesulphuric acid, distils off. The vapour passes off bythe stoneware arm, e, into the leaden draught-pipe,F, where a quantity of it condenses; but the greaterportion finds its way from thence into the chimney.The contents of the retort are kept boiling for somehours, or until the brown colour entirely disappears;for this appears to be the only reaction by which todetermine the conclusion of the rectification. Itwould appear that the organic matter resists thedestroying or oxidising action of the acid, until ithas reached its highest point of concentration andtemperature. When the clearing has taken place,the fire is withdrawn from beneath the pot; andthe whole arrangement is allowed to cool down to asafe temperature, when it is drawn off in carboys. Thisdrawing off is effected by a siphon, with the shorter ofits two legs turned back upon itself. In order towithdraw the concentrated acid from the retort theworkman first removes the earthenware arm; thesiphon is filled with water, the man keeping histhumb firmly closed over the orifice of the longerleg. The shorter leg is inserted into the retort,and gently placed on the bottom; the workmanthen removes his thumb for a few seconds, untilthe acid has driven before it the water with which thesiphon was charged; when this is done, the end isdirected into a carboy which has been placed inreadiness by an assistant. When the carboy is fullthe orifice is again closed by the thumb, while thefull carboy is removed and replaced by anotherempty one. The whole range of retorts are emptiedin this manner.

The product is the strongest English sulphuricacid; it is colourless or nearly so, and is of thedensity 1-850, or 170° on Twaddell’s hydrometer.

In 1871 an entirely new process was devised byJ uni us Gridley of New York , and was patented in thiscountry as a communication by Henry Chance, of thefirm of Messrs. Chance Brothers, & Co., of Birming-ham . To the ordinary system of working with glassretorts, unconnected with each other, in which therectifying is commenced, continued, and finished inone retort (as described above), there exist manyobjections. The acid in each individual retort, afterbeing rectified, has, together with the surroundingbrickwork, to be cooled down before it can be drawnoff into carboys. This necessitates a large consump-tion of coal in reheating the brickwork each time theretort is filled with crude acid. In bringing the acidto the boiling point at the commencement, and incooling it down again at the close of the operation,the very greatest care has to be taken to avoidbreakage; while the liability to accident, and thelabour required in filling and emptying each retort,are material objections to the process.

Under the new system these difficulties are obvi-ated by making the process continuous. The retortsare set in beds of four each. They are arranged,as shown in Fig. 13, on different levels, and are