374 DIXON AND TAYLOR : APPARATUS FOR DEMONSTRATINQXLI.-Apparatus for Demonstrating the Electrolysisof Hydrochloric Acid.By AUGUSTUS EDWARD DIXON and JOHN TAYLOR.THOSE who have occasion to use Hofmann’s apparatus for showingthe electrolytic decomposition of hydrochloric acid into equalvolumes of its constituent gases, soon become aware that its com-parative simplicity has been attained only a t the cost of somedisadvantages. Of these it is sufficient to mention (i) the leakageof the acid electrolyte at the bottom of the H-shaped tube; (ii) adifficulty in ascertaining when the saturation with chlorine iscomplete ; and (iii) the possibility, in certain circumstances, ofthe two gases becoming mixed. Various devices have been proposedto obviate these troubles; they are, however, not very satisfactoryin practice, and in any case, when it comes to the final demonstriltion, there is always distinct inequality between the volumes ofthe liberated gases.In L.Meyer’s improved apparatus (Ber., 1894, 27, SSO), thesimplicity is abandoned ; for, whilst Hofmann’s original form ofvoltameter is retained, the gases are delivered apart, underdiminished pressure, into separate tubes, one of which, containingwater, serves to collect the hydrogen; the other, filled a t firstwith saturated chlorine-water, receives the chlorine ; each of thesetubes, open below, stands in a trough of the liquid with whichit is charged. Over the earlier form Meyer’s modification has onedistinct advantage, since by means of it the equality in volume ofthe resultant gases may be demonstrated; on the other hand, notonly must saturated chlorine-water be prepared, and the collectinTHE ELECTROLYSIS OF HYDROCHLORIC ACID.375tube filled with it, but also the exposure, on the lecture-table, ofan open dish charged with this liquid is not free from objection.Moreover, the three disadvantageous features mentioned above arenot eliminated.The apparatus here figured, although less simple than that ofHofmann, is sufficiently compact to be set up on a single stand,and in practice has given results that are satisfactory. Briefly,it consists of two parts: A, the electrolyser, and B, the vessel forAreceiving and measuring over concentrated sulphuric acid the gasesdelivered from A.The electrolyser is a U-tube of 1-inch bore, having near each enda delivery tube, as shown; a well-paraffined cork in each neckcarries a half-inch carbon cylinder, one foot long; to the projectingfree end of this rod a brass binding screw is clamped.The collecting and measuring vessel is a tall U-tube, of somefiveeighths to three-quarters of an inch in bore and about 1376 ELECTROLYSIS OF HYDROCHLORIC ACID.inches long, to the lowest point of which another tube (thepressure-tube) is sealed as shown, to end in a bulb above thehighest point of the U ; a t the foot of the pressure-tube, just whereit begins to be,nd upward, is sealed on a light glass tap, having itsaxis parallel to the plane of the U-tube.Into the top of eachmeasuring tube is ground a hollow glass stopper, which terminatesin an obliquely bored two-way tap, communicating at will, either.with a short stand-tube of about one-eighth of an inch in boreand an inch or so in length, or with the bent receiving tube, justalongside; the distance between the latter and its fellow of theopposite side is such that each is in a, straight line with thecorresponding delivery tube of the electrolyser.By suitable pieces of glass and rubber tubing, the two mainparts are connected as shown, the hydrogen-connexions being madewith butt-joints and thick-walled rubber tube, well smeared insidewith glycerol; between the chlorine delivery tube and the corre-sponding receiving tube a T-piece may conveniently be introduced ;this, when provided with rubber connexion, pinchcock, and glassdelivery tube, serves to pass the waste chlorine, when desired, tothe table-draught, or into a beaker of lime.When the electrolyser is filled, the electrolyte may reach to withinan inch of the delivery tubes.The receiver is charged in allthree limbs to the level of the bottoms of the stoppers,. whereground in to the measuring tubes; the sulphuric acid used forfilling may be stained, if the operator wishes, by a, method givenbelow. The parts are now connected; the pinchcock is opened,both gas-taps are turned into position for receiving, and the plugof the one for hydrogen pulled out sufficiently to allow this gas,when liberated, to pass freely out from the electrolyser into theair.The current is now turned on, and maintained until the chlorineis seen to be escaping freely.To ascertain if saturation is complete,the pinchcock is closed, the chlorine-tap pulled out for a momentfrom its seat to equalise pressure, then both taps are pushed homesimultaneously, and the lowest tap is turned on, so that the gasesmay accumulate in the collecting tubes under a pressure notgreater than the atmospheric, or less, as may be desired. There isno difficulty in knowing if saturation is complete, for, when thisstage is attained, the two tubes fill at exactly the same rate fromstart t o finish.To recharge the collecting vessel, the pinchcock is opened, theplug of the hydrogen-tap pulled out as at first, and the sulphuricacid, which was drawn off, is returned to the apparatus throughthe bulb a t the top; after this the procedure is as before.WheSOLUBILITY OF POTASSIUM SULPHATE. 377the production of the two gases in equal volumes has beendemonstrated, the acid may be returned to the bulb, and t.he gases,displaced through the two short stand-tubes, may be proved, inthe usual way, to be hydrogen and chlorine respectively.It is scarcely necessary to mention that the slowness (one canhardly call it speed) of saturation with chlorine varies considerably,according to whether the electrolyte is hydrochloric acid alone, thesame saturated with salt, or saturated brine; in every case thetediousness is, of course, much reduced by preliminary saturationof the electrolyte with chlorine.The gas-taps shouldbe smeared with vaseline as lightly as will suffice to render themgas-tight; otherwise more or less chokage may occur; the butt-joints, etc., are recommended for the hydrogen connecting tubeson account of the facility with which this gas escapes throughrubber; and the cork joints, of course, must be made tightwith paraffin or other suitable luting; also, the carbon rods,when done with, should be well washed and dried, to preventdisintegration.It is not easy for persons sitting a t a distanceto see clearly the accumulation in a narrow tube of colourless orf aintly-coloured gases over a colourless liquid. With the apparatushere described, this difficulty may be overcome by dissolving inthe sulphuric acid enough chrome-alum to stain it deep green;if between the collecting tubes and the pressure-tube a sheet ofmilk-glass be interposed, with a light close behind, the filling ofthe former is rendered easily visible from any part of an ordinarylecture-room. The glass parts of this apparatus were made byMessrs. Baird and Tatlock, in accordance with drawings suppliedto them.I n conclusion, a few points may be noted.UNIVERSITY COLLEGE,CORK