. Manual of physico-chemical measurements. acetic acid, are very hygroscopic,and consequently change their melting point by absorbing traces ofwater during an experiment. As the molecular weight of water is verysmall, these traces have a particularly large influence on the freezing point. To obviate thisdifficulty Beckmann hasproposed the arrange-ment exhibited in The stirrer passesthrough a widened tubethrough which dry airis led during the ex-periment, escaping abovebetween the rod of thestirrer and the tubeenclosing it. A fewdrops of concentratedsulphuric acid are con-tained in the
. Manual of physico-chemical measurements. acetic acid, are very hygroscopic,and consequently change their melting point by absorbing traces ofwater during an experiment. As the molecular weight of water is verysmall, these traces have a particularly large influence on the freezing point. To obviate thisdifficulty Beckmann hasproposed the arrange-ment exhibited in The stirrer passesthrough a widened tubethrough which dry airis led during the ex-periment, escaping abovebetween the rod of thestirrer and the tubeenclosing it. A fewdrops of concentratedsulphuric acid are con-tained in the bulbs F ;a cross plate in thelarge bulb prevents itfrom spirting over. Theair is got from a waterblowpipe, and dried ina special column forthe purpose. ^ is arubber handle, H asiphon to remove theliquid from the bottomof C when necessary,and G a metal dish, inwhich the apparatusstands, for receiving the Fig. 130. liquid^ When the substancea solid, it is introduced in the form of powder contained in a 1 Zeit. physik. Cham., 7, 324, XIV DETERMINATION OF MOLECULAR WEIGHTS IN SOLUTIONS 183 long narrow test-tube, so that it can be shaken directly into thefreezing vessel without any of it falling on the side tube. Thesmall testtube is weighed before and after the experiment. Liquidsare introduced from a pipette of the formshown in Fig. 131, the same mode of proceed-ing being adopted. It is always advantageousto make several determinations in succession,fresh substance being introduced on the con-•clusion of the first measurement, and a newmeasurement then made. The result is calculated by means of theformula— M=K-
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