ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 129 
high maximum, and then descended very gradually to a fixed lower reading. 
The pincette was traversed to and fro over the rock-surfaces while the 
_ thermometer was being noted, and exhibited during these motions fluctu- 
ations answering to about one or two Fahrenheit degrees on either side of 
an average position; corrected for zero of the scale, and reduced by trials 
for this purpose between every two or three experiments to Fahrenheit 
degrees, the temperature-difference thus found, divided by the quantity of 
heat transmitted to the cooler per minute, gave the apparent thermal con- 
ductivity of the plate. The results, in Peclet’s units, were scarcely more 
than one third of what Peclet and other earlier experimenters had obtained. 
It was obvious that instead of marking the temperature-difference between 
the two solid contact surfaces of the rock and velvet which they touched, 
the points of the thermoelectric forceps showed the temperature of the fluid 
air-bath in which those two Surfaces are immersed. The extreme mobility of 
this fluid medium, enabling it to pass to and fro through the velvet between 
the plates of the heater and the cooler, while it equally insinuates itself 
_ between the rock-surface and the thermopile that can only enter into actual 
solid contact with each other (at least theoretically) at three points, controls 
the temperature of the metallic thermometer far more powerfully than the 
rock-face that it touches, and the real temperature-differences between the 
rock-faces are accordingly completely masked. It is very probable that if 
the velvet covers on the instrument are replaced by caoutchouce or soft wash- 
leather, the source of this error will be very much reduced ; and although it 
is certain that the confronting rock and leather surfaces will never have 
actually the same temperature from the existence of a sensible quantity of 
resisting air between them (so that, as before, the thermopile will not mark 
the true rock-temperature-difference, but a mean between that difference 
and a similar difference for the leather-faces), yet the range of this error 
will be considerably smaller than in the experiments already made with 
velvet covers, whose loose texture makes air-currents the principal medium 
of heat-transmission through them. The comparative results now obtained 
are accordingly only subjoined with this Report as first approximations, 
from which the crrors, anticipated last year as likely to arise from surface 
characters of the rock sections, are as yet far from having been satisfactorily 
removed. 
To obtain the true rock-temperature-differences means were taken to 
cement the thermopile-points to the rock with plaster, which it would be 
desirable to adopt with as few samples as possible, on account of the tedious- 
ness of the process, and the injury from using them thus .as standards of 
correction for the rest done to the beautifully worked surfaces of many of the 
plates. If the correction found to be required ‘can be restricted by the 
mode of operating to a range of such small limits as to be applicable gene- 
rally, without appreciable influence of the surface characters in making its 
occasional departures from a mean value very sensible, then the reduction- 
factor, found by absolute experiments with a few rocks of characteristically 
rough and smooth or polished surfaces giving the true temperature- 
difference for a given heat-flow from the apparent one shown by the thermo- 
couple placed simply between the rock and leather faces, will be admissible 
(within the limits of error of the observations) to convert a list of apparent 
conductivities, as just supposed to be obtained, from a mere comparative 
table of relative conducting-powers to a table of absolute thermal conducti- 
vities, in which the errors of the values given will certainly not be greater 
than i. in all probability have been committed had the direct method 
1874. K 
