508 
MR. 0. H. LEES ON THE THERMAL CONDUCTIVITIES 
the present paper agree fairly, the only great difference being in the case of Iceland 
spar along the axis. 
Meyer’s determinations depend on the rate of rise of the temperature of a calori¬ 
meter into which a heated cube of the material is plunged. It is questionable whether 
the assumption which he makes, that the temperature of the surface of the cube at 
any instant is that given by the thermometer in the calorimeter, is justifiable on 
account of the difficulty of preventing a layer of water adhering to the surface. The 
same objection applies to the method which he used to confirm the results obtained by 
his first method. 
Remarks. 
From the Table of Results it is at once evident that, for transparent bodies, no such 
comparison between thermal conductivity and velocity of propagation of light can be 
made, as has been made by Kundt for the metals. The thermal conductivities vary 
enormously for very small change of refractive index, and the variations are sometimes 
in the same direction as the variation of the index of refraction, sometimes in opposite. 
It is remarkable that the bodies quartz and rock salt, which are diathermanous 
bodies, should also be good thermal conductors, quartz being a better conductor than 
bismuth. To prove that the high value of the conductivity is not due to heat passing 
through these bodies by radiation from the hot bar to the cold, several experiments 
were made with the apparatus arranged as it was during the experiment in quartz, 
but the quartz disc was removed, thus allowing the hot bar to radiate heat through 
the intervening air space to the cool bar. Under these conditions, no change of 
temperature at the point of observation near the end of the cool bar, could be 
detected when the hot bar was suddenly cooled. As air is a more diathermanous 
body than quartz, it is thus evident that the amount of heat radiated through the 
quartz from one bar to the other, is too small to affect the above results. 
The high conductivity of quartz would render the use of fused quartz (the conductivity 
of which does not probably differ much from that of quartz crystal) advantageous for 
vessels subject to sudden change of temperature, and for delicate thermometers. 
The results for Iceland spar and marble seem to indicate that the irregular arrange¬ 
ment of the crystals in marble interferes with the passage of heat. 
Water and salt solutions have conductivities about equal to that of glass. 
The solid insulations used in electrical work rank with glycerine and the oils, so far 
as thermal conductivity is concerned. 
Silk has about one-third the conductivity of shellac. Hence, if a silk covered wire 
has the covering saturated with shellac, it will have its heat conducted away to 
surrounding bodies much faster than previously. The practice of soaking galvanometer 
coils in shellac is, therefore, good from a thermal point of view, as it enables the heat 
generated in the wire to pass more rapidly to the exterior of the coils, and be 
radiated away. 
