HEAT OF EVAPORATION OF WATER. 299 
shown in figure) containing the same depth of H,SO,as the bulbs to be weighed. 
Thus when T, was afterwards so turned as to change the direction of the current to 
the weighing bulbs, no alteration tock place in the rate of flow, which was adjusted as 
required in the following manner. A T-piece (not shown in sketch) was inserted in the 
tube between the drying tower P and the tap T;. The vertical leg of the T-piece was a 
wide graduated tube, open at its lower extremity, and immersed in a deep, narrow 
vessel (a hydrometer jar) filled with H,SO,. If the air pressure within the pipes was 
greater than that due to the column of H,SO, above the open end of this leg, the 
excess of gas bubbled up through the H,SO, and escaped into the open air. Thus by 
raising or lowering the vessel the pressure and quantity of air passing through the 
whole apparatus could be regulated with exactness. I found it possible to so adjust 
the flow that when the heat supply was Q. and Q,, the galvanometer showed no 
change for nearly an hour. A further advantage of this arrangement was that I could 
close any tap without increasing the pressure within the apparatus, the whole of the 
gas then passing out through the regulator. The pressure of the gas near the flask 
was read on the manometer M,, which had two wide bulbs near the base, the mercury 
filling the lower bend and three-quarters of the bulbs. The narrow tube on the scale 
side contained water resting on the mercury, and thus a very open scale was obtained. 
The instrument was carefully standardised, and a movement of 7°78 centims. in the 
water level corresponded to a change in the pressure of 1 centim. of mercury. The 
water was completely cut off by the mercury from all] communication with the gas 
tubes. 
The electric connections having been made and the rheochord adjusted until the 
high resistance galvanometer (G,) showed that the D.P. at the ends of the coil was 
that of the Clark cells (I shall speak of this hereafter as the “ electric balance,” and 
of the adjustment of the loss or gain of heat as the “thermal balance”), the gas flow 
was then diminished until the platinum thermometer galvanometer showed that 0, 
slightly exceeded 4, (#; = calorimeter temperature, 6, = temperature of surrounding 
walls). The current was then switched on to the alternative coil F (fig. 3), 6, now 
decreased and at the moment when 6, = 6, the current was switched back again, 
the key itself recording the time on the chronograph tape. About the same time, 
the 4-way tap T, was turned so as to cause the escaping gas to pass through the 
weighing bulbs. This time had not to be recorded, nor was there any necessity that 
it should be nearly or at all coincident with the time of establishing the electric 
current. If, however, 6, did not exactly equal @, at the moment of turning T,, the 
swing of the galvanometer was recorded and the tap only closed at the end of an 
experiment when the swing was identical with the initial one. 
When necessary the thermal balance was maintained for 1, 2, or, in some cases, 
3 hours by adjusting the flow of gas. At the close of an experiment 0, was again 
allowed to rise slightly above 6), the current: switched off (again itself recording the 
time) and as @, descended the tap was turned back to its old position when 0, = 6,, 
2Q 2 
