presence of which would vitiate the results of the trial. To allow the piston to be withdrawn without any shock, or the danger of dividing the column abruptly, the rod attached to it is made hollow throughout. In using it the operator applies the ball of his forefinger to the top of the piston-rod (_E_), in order to create a vacuum as he raises it; and then withdraws it, to readmit the air when he thrusts it down or removes it from the tube. The excess of liquid (if any) in the pipette is then run off until its upper surface is exactly level with the zero (0) of the scale, at 25° C., to which it is raised by immersion in a water bath of that temperature, as observed by the thermometer; which is done by very cautiously turning the rod which depresses the valve. The whole apparatus is now again immersed in the water bath; and, held by the upper portion of the plate, kept in gentle motion with the hand, until the temperature rises to exactly 50° C., when the coefficient of expansion is obtained, and hence also the proportion of alcohol——the scale of the instrument being so graduated, from actual experiments previously made upon mixtures of known composition, as to give, at once, the per-centage of alcohol by VOLUME (nearly).[14] [Footnote 14: ‘Comptes Rendus,’ xvii, 418.] [Illustration] _e._ From the TENSION of the VAPOUR:——Geissler’s ALCOHOLOMETER. This method, for which we are indebted to M. Geissler, of Bonn, depends on the measurement of the tension or elastic force of the vapour of the liquid, as indicated by the height to which it raises a small column of mercury. The spirit, wine, or other liquor, of which it is desired to ascertain the strength, is put into the little flask (_a_), which, when completely filled, is screwed on to the curved glass-tube which contains the mercurial column (which is inverted for the purpose), and is closed by the stop-cock (_b_). The instrument (see _engr._) is then placed erect, and the flask and lower part of the tube immersed in a water bath, as in the previous method. The number, on the graduated scale of the instrument corresponding to the height of the mercury, at the boiling point of the liquor under examination, gives the per-centage of alcohol by VOLUME (nearly). This method furnishes approximative results with great facility and expedition; and, with proper care, these do not vary more than 1/3 to 1/2 of 1%, from those obtained by distillation. We find, that by having the diameter of the part of the tube at which the surface of the mercury is acted on by the vapour a little larger than that of the longer limb, and by previously abstracting the air from the sample, as in Silbermann’s method, or even by agitation and exposure in an open vessel, the two may be made to correspond almost exactly.