TRANSACTIONS OF THE SECTIONS. 15 
tions the more diathermic liquids exhibited a remarkable and anomalous deport- 
ment towards radiant heat. ‘This observation led him to make a subsequent investi- 
gation, the results of which are summed up as follows :— 
When the more diathermic liquids are introduced between two parailel plates of 
rock-salt separated by a very small interval, the rays from an artificial source are 
found to be more freely transmitted than when air intervenes between the plates. 
For a space of ‘02 inch wide the increased transmission amounts with bichloride 
of carbon to about 12 per cent., with bisulphide of carbon to 9 per cent., and with 
chloroform to 4:5 per cent. This effect disappears and less heat is transmitted (1) 
when the transcalency of the liquid diminishes; e. g. the same thickness of sul- 
phuric ether intercepts 30 per cent. of the heat previously passing through the 
empty cell; (2) when the distance between the plates is increased beyond, say, 
7, of an inch in the case of bisulphide, and +}, of an inch in the case of bichloride 
of carbon. The increased transmission by these liquids reappears, however, in 
thicker layers when plane parallel glass plates are substituted for rock-salt, and 
continues, apparently indeed augmenting, as the depth of the cell increases, so 
far as the experiments were carried. Bisulphide of carbon, poured into a cell with 
glass sides 1-2 inch apart, increases the heat falling on the pile 6 per cent., and 
bichloride of carbon a still larger amount. Altering the temperature or nature of 
the source, the size of the aperture ina screen behind the cell, or the position of 
the cell, makes no material change in these results. But altering the character, 
or augmenting the thickness, of the walls of the cell has considerable influence. 
For example, if the cell-walls be of glass, increasing their thickness from one- to 
three-tenths of an inch raises the heat falling on the thermoscope 6 per cent. when 
equal depths of the selfsame liquid are poured into the cell. Again, by merely 
changing the parallel sides of the same cell from rock-salt to precisely similar plates 
of glass, the very same liquid can be shown to intercept a certain quantity of the 
heat falling on the thermo-pile in the one case, and to augment that quantity in 
the other—the difference amounting to upwards of 10 per cent. of the total radiation 
through the empty cell. 
The explanation of the foregoing facts may be traced to two main causes. The 
increased transmission noticed with films of the more diathermic liquids chiefly 
arises from the reduction or abolition of the reflection taking place from the interior 
surfaces of the walls of the cell, owing to the optical density of the liquid intro- 
duced being nearer to the cell-walls’ than that of the medium it replaces. But in 
glass cells of considerable depth, retaining the former explanation, the augmented 
heat there observed is probably mainly due to an effect of the refraction of diver- 
gent rays by plane surfaces; this gives rise to a concentration of the beam, which 
become sensible when accompanied by a great transcalency of the liquid in the 
cell. In similar cells with rock-salt ends the effect is not observed, probably on 
account of such cells sifting the beam far less than glass, and thus permitting a 
higher absorption of the liquid. Nevertheless even with rock-salt cells the causes 
alluded to must necessarily render, to a certain extent, incorrect the precise absorp- 
tion hitherto attributed to liquids. These sources of error in determining the true 
absorption of a liquid or solid can, however, be avoided by employing truly parallel 
rays; and these are best obtained from the sun. 
On the Thermal Resistance of Liquids. By FrepEericx Gururiz, F.R.S.L. 
Tf we wish to get an insight into the specific resistances of the elements and into 
the law connecting thermal resistance and chemical constitution, we must examine 
liquids rather than solids, because while the former are essentially homogeneous, 
the latter are never without structure, and seldom even without texture. 
To examine the conductivity of a liquid, it must be either heated from above or 
cooled from below, in order that convection may be avoided. If the liquid be 
contained in a vessel, the difference between the conductivities of the liquid itself 
and of the containing vessel will also introduce convection. In spite of the 
labours of many able physicists, these difficulties have hindered the prosecution of 
