FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OF LIQUIDS. 641 
to solids, and which would be as free as possible from the disturbing influences already 
pointed out. 
§ 20. The extreme sensitiveness of the thermo-electric pile cannot be made use of 
directly in experiments of this nature. The pile leaves little to be desired where mo- 
mentary effects of radiant heat are concerned ; but it is obviously inapplicable where the 
quantity of heat has to be considered, as must be the case wherever time is an element 
of the result. 
§ 21. After many trials I have adopted, for the present, the form of instrument de- 
scribed below, and which I believe will be found to answer for a great variety of condi- 
tions. The instrument may be called a “ Diathermometer.” Although it has been 
hitherto employed exclusively for liquids, it is equally applicable to solids. The appa- 
ratus has been skilfully constructed by Mr. W. Ladd, of Beak Street, London. The ex- 
periments were made partly in the theatre of the London Institution, and partly in the 
cellar-passage beneath it. Two conditions essential to satisfactory results, are freedom 
from vibration and uniformity of atmospheric temperature — conditions difficult to ob- 
tain in our metropolis and climate, but which were, I think, as nearly as possible satis- 
fied by the solid foundation of the building and the large volume of still air, in or beneath 
which the experiments were performed. 
§ 22. Plate LXV. fig. 2, shows the diathermometer in almost actual size in elevation. 
Plate LXV. fig. 3 is a view of the entire arrangement of the apparatus. The whole of the 
diathermometer is of brass, with the exception of the bases of the cones to be described 
immediately. Upon a mahogany slab, U (fig. 2), there rests a tripod stand, S, supported 
on two screws, T, T, and the point N. From one end of the stand above N there is fixed 
the triangular vertical prism M, which terminates above in a very accurately cut screw, 
K K. Exactly fitting M is a sliding collar, H, to which is screwed the bracket G. The 
arm F is screwed firmly upon G. One end of F is pierced so as to allow the screw K K 
to pass through and is clasped by the nut I J, which works accurately upon the screw K K. 
The head of the nut I, where it is in contact with the arm F, is graduated into lOths 
and lOOths. Upon the arm F, beneath the graduation of the nut, is an arbitrary mark, R. 
The other extremity of F carries a collar, into which is screwed the brass cone A, also 
provided with a collar, W. The cone A is made of sheet brass, and has the size and 
shape shown in the figure. Its lower or flat surface is covered with a thin sheet of 
platinum, which is soldered to the brass. The diameter of the platinum base is 53*34 
millims. The neck of the conical flask (A) is provided with a cork (E), through which 
pass two glass tubes, C and D. The tube C reaches to 0*5 millim. from the bottom of the 
cone. The tube D is open immediately beneath the cork. Exactly facing A is a per- 
fectly similar cone, B, which is screwed into the centre of the tripod stand, S. The lower 
cone is also faced with platinum. The neck of B is furnished with a cork, P, through 
which passes the vertical tube Q, open at both ends. This portion of the apparatus 
must be scrupulously air-tight. The lower end of Q dips into a wide vessel of coloured 
water, and carries a paper scale divided arbitrarily, but into exactly equal divisions. 
