650 FREDERICK GUTHRIE OK THE THERMAL RESISTANCE OE LIQUIDS. 
quick one. In fig. 6 the points of the curve 1 represent the variation of t with AT ; 
those of 2 represent the variation of t with e). 
VI. The Quantity of Heat conducted, or Measure of Resistance. 
§ 48. In order, by means of the expansion of the air in B, to measure the quantity of 
heat which passes through a liquid under given conditions, or to measure the resistance 
which the liquid offers to the passage of heat, it is necessary to know the capacity of the 
lower cone B, and the calibre of the different portions of the tube Q, together with the 
whole pressure on the air in B. A Table was formed, which being arbitrary need not be 
here given, showing the capacity in cubic centimetres of the different parts of Q, which 
was slightly conical. The lower cone B and a small portion of Q, as far as a certain 
point, had the volume 45 cub. centims. 
§ 49. It is clear that the increase of volume in B does not directly give the quantity 
of heat which passes through the liquid L, because part of that heat is expended in 
heating the metal of the lower cone ; and moreover the heated air in the lower cone 
accumulates towards the top of the lower cone, so that although the coefficient of 'air- 
expansion is virtually the same at all temperatures, yet there being less temperature-dif- 
ference between the platinum and the air than would be the case if the air in cone B 
were continually mixed, the ingress of heat must be smaller and the thermal resistance of 
the liquid would appear too great. A certain amount of radiation from the lower cone 
tends in the same direction, as also does the loss of heat due to its expenditure in pene- 
trating the cone bases. It would be nearly impossible to estimate and allow for all 
these errors; but they may be eliminated en masse. For if the cones are in the first 
instance brought into actual contact, and this expansion in B is measured under given 
experimental conditions, and if then the expansion is also measured when a liquid is 
inserted under like conditions, then the difference between the two effects is a true 
measure of the thermal resistance of the liquid, because the same errors being attached 
with the same signs to both determinations, do not affect their difference. 
§ 50. The platinum faces of the cones were brought into actual contact by wetting 
their faces with mercury'*. The quantity of mercury so introduced was immeasurably 
small, and its presence causes a far less error than would attend the presence of an air- 
film. It was found .that the value of t in all cases was too small for measurement, in- 
deed the transmission of heat appeared instantaneous; it was certainly less than 0 S, 5. 
In these, as in the previous experiments, I have preferred to give the nearest whole 
number of seconds rather than to attempt to subdivide a second. The expansions were 
measured after 1, 2, and 3 min. The necessary corrections have been made in these 
numbers for the varying pressure in B. 
* Eor a method of wetting platinum and other metals with mercury, see a paper hy the author on Drops 
(Royal Society Proceedings, 1864). When the cone-faces are wetted with mercury and the one face is slid upon 
the other, the lower cone may he weighted with from ten to twelve pounds before the two are separated. 
