644 FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OE LIQUIDS. 
Thus, in order to get in A a temperature 10° above the atmospheric temperature, T, 
the water in 1 must be heated to T + 10*28. To get an excess of 20° in A, the water in 
1 must be heated to T+20’65, and so on. From these three values, which include 
almost the entire range of temperatures which I have employed, the intermediate values 
can be interpolated when required. These values may vary slightly with the atmo- 
spheric temperature at the time, but such variation is within the limits of observational 
error *. 
§ 32. Radiation . — The question is a vital one, whether, and if so, what proportion of 
the effect produced in the lower cone is due to radiation through the liquid. If radia- 
tion takes place in such a manner as to completely penetrate the liquid and reach the 
lower platinum surface, the effect would inextricably complicate the phenomena ; for while 
some of such radiant heat would be reechoed upwards and downwards between the cones, 
a portion would be absorbed by the lower plate, and this would not only augment the 
expansion of the air in the lower cone, but being shared by the lower part of the liquid 
(L) in contact with it, currents would be formed and the results vitiated. Such effect 
might indeed be to a great degree diminished by making the face of the lower cone of 
fluor-spar or even of glass. But the great thermal resistances of such substances, com- 
pared with that of a metal, would more than counterbalance the possible benefit to be 
derived from their greater diathermancy. 
§ 33. Actual experiment, however, showed that the effect of radiation was inappre- 
ciably small. 
§ 34. It will be admitted that radiant heat would pass with a velocity so great that 
the time-lapse between its emission from the upper plate and its effect upon the lower 
cone would be inappreciably small. The following results with water show that, under 
the following conditions, the time-lapse between the influx of water into A and the 
commencement of the depression in Q was pretty constant and considerable. 
T=Temperature of air or initial temperature of liquid =21 0- 39 
Tj= Temperature of water in A = 31°*39 
AT = Temperature-difference - =10 o, 00 
£=Time required to produce first effect in Q. 
S= Thickness of liquid, L. 
$= 
2 millims. 
s 
$=3 millims. 
s 
(1) 
t=6 
t — 10 
( 2 ) 
t= 7 
t = 10 
( 3 ) 
t= 7 
t = 11 
( 4 ) 
t = 5 
t = 11 
( 5 ) 
t — 7 
t = 10 
Mean 
<=6-4 
#=10*4 
* Strictly the temperatures in 1 should he observed when the temperature in A is fixed ; the difference be- 
tween the results so obtained and those given is vanishingly small. 
