260 ME. V. H. VELEY ON THE CONDITIONS OF THE 



from the same piece of glass tubing ; the two limbs of each are in close contact, being 

 wired together above and below. Joined to the centre of the upper part of one of 

 these tubes is a stopcock, through which, by means of flexible tubing connecting it 

 with an aspirator, the collected gas can be drawn out till the tube is filled with water 

 when the open ends are standing in a pneumatic trough. Into one limb of this tube 

 the gas is delivered in minute bubbles from the open end of a capillary tube, joined 

 to the decomposition flask ; in the other limb the unbroken surface of the water falls 

 steadily. The twin tube contains a quantity of air, measured once for all ; the level 

 of water in one limb serves to mark the point at which an equal volume of gas has 

 been collected in the other tube, the level of water in the adjoining tubes being the 

 same. The actual volume varies with conditions of temperature and atmospheric 

 pressure, but, since the contents of both tubes are under precisely the same conditions, 

 this variation is immaterial. 



The standard quantity of air taken throughout these experiments was 10 c.c. of 

 dry air under standard conditions of temperature and pressure. This was measured 

 out as follows : The volume occupied by 10 c.c. under the conditions of temperature 

 and pressure on the day of measurement was at first calculated ; this was corrected 

 for diminution of pressure due to tension of aqueous vapour and the dip of the 

 delivery tube under the surface of the water. This volume was delivered into the 

 U-tube by means of a capillary tube, from a flask in which that volume of air was 

 displaced by water dropping from a burette. The temperature of the flask was kept 

 constant by immersing it in a vessel of water ; while the effect of draughts was 

 minimised by using the capillary tube. In reactions in which a gas insoluble 

 in water was formed, the small volume of gas evolved between each observation 

 of level and the refilling of the tube by means of the aspirator was collected in a 

 separate tube (C, fig. 1), and decanted up during the interval between this and the next 

 observation. Thus the whole process was continuous. In cases in which the gas was 

 partially soluble in water, suitable modifications were introduced or different methods 

 of collection adopted. The observations were made with the help of a clock, beating 

 seconds, stationed close to the apparatus ; and, by counting, the exact second could be 

 noted at which the slowly descending water was at the same level as the similar 

 column of water in the twin U-tube. The flask in which the reaction was carried on 

 was heated in an air or water bath, the temperature of which was kept constant by 

 means of an automatic mercury and oil gas-regulator ; in most cases the variation of 

 temperature did not exceed one- tenth of a degree. Long-range thermometers were 

 used, graduated to tenths of degrees, in which differences of a twentieth of a degree 

 could easily be noted ; some of the thermometers were calibrated according to the 

 method of BESSEL. 



