304 MR. E. H. GRIFFITHS ON THE LATENT 
possessed the following great advantages: (1) there would be no necessity for the 
passage of gas through the apparatus, (2) the weighing-bulbs, &c., for catching the 
aqueous vapour could be dispensed with. I was, however, reluctant to adopt this 
method, for I anticipated that several practical difficulties would present themselves ; 
for example, it would be necessary to insert a known weight of water into the flask, 
and continue the experiments until the whole of that water was evaporated, thus I 
should not be able to finish the experiment at any time when 6, = 6), as was the case 
during the “ pressure ” experiments. 
It would also be more difficult to maintain the thermal balance at such perfection 
as in the former experiments, for it would be less easy to control the rate of 
evaporation. 
Again, when air was passing through the apparatus, the water in the flask was 
efficiently stirred and its temperature in consequence approximated closely to that of 
the calorimeter, even when the rate of evaporation was rapid. A thermometer (G,) 
was, during the preliminary experiments, placed as described in Section V., with its 
bulb near the bottom of the flask, and I found that the difference in temperature 
between the flask water and the calorimeter liquid did not, in the greatest case, exceed 
0°15 C. If no gas was passing, the water would not be stirred, and it appeared 
probable that the difference in temperature might greatly exceed this amount. In 
this case not only would the observed temperature (0,) exceed the real temperature of 
the water, but also the vapour when escaping through the coil would abstract heat 
from the calorimeter, and thus (as pointed out by WINKELMANN in his criticisms of 
REGNAULT'’S experiments) the resulting value of L would be too great. 
It did not appear possible to determine the interior flask temperature accurately by 
means of a mercury thermometer, whose bulb was placed in a chamber where the 
pressure was reduced to something between 10 and 100 millims., and if I placed a 
platinum thermometer in the flask, a third electrical circuit and galvanometer would 
have had to be added with a third observer to read the indications—additions which 
circumstances did not render possible. 
I did not feel that it would be of any use to adopt RecNauut’s method of 
determining the temperature by the vapour pressure (see Section II.), and I 
endeavoured therefore to find some means of preventing the temperature of the 
evaporating water from falling below that of the walls of the flask. 
It was evident that the smaller the quantity of contained water, the more nearly 
would its temperature approximate to that of the surface upon which it rested. If 
it was possible to discharge the water drop by drop on to the silver surface, the 
difference in temperature would probably be negligible, and after some trials I found 
a method of effecting this. A glass tube, exactly fitting into the communicating 
tube h h’ (Plate 5, fig. 1) was closed at one end, the other end being drawn out so 
that it would pass through the constriction at the calorimeter lid (see p. 279), and the 
narrow tube which thus projected into the flask, terminated in a very fine opening. 
