67 
of Edinburgh , Session 1872-73. 
The paraffin is that kind which has its boiling point at 45° C. 
The sand was very fine, nearly pure silica, being that used for 
sand-baths. 
The sawdust was that of common firwood, and was compressed. 
The flannel was of the very coarse kind usually known as wash- 
ing cloth. 
The coarse linen was of the coarsest possible texture. 
The quartz used for conduction along the axis was very thin. 
The piece used for conduction perpendicular to the axis was a large 
piece in the form of a hexagonal prism. 
With regard to the numbers, I must say, in the first place, that 
they differ considerably from those of Peclet in nearly all the cases 
that admit of comparison. Reducing his numbers to the units 
employed above, we find — 
Substance. 
Forbes. 
Peclet. 
White marble 
•0691 
•463 
Glass 
•0300 
T25 
Carbon . 
•0243 
•827 
Caoutchouc . 
•00534 
•028 
Sawdust 
•00735 
•Oil 
Cotton wool . 
•00530 
•00666 
It appears that there is a constant error due to the difference of 
methods. But it may be well 
to remark that Peclet has found 
very different results at different times, as may be seen at once by 
comparing the tables given at pages 355 and 481 of his “ Traite de 
la Chaleur,” 1843, vol. ii ., and at page 406 of the second vol. of the 
same work as published in 1861. My experiments have at times 
varied, and I have given all the values I obtained for quartz, to 
show how injudicious it is to use a thin piece of a substance that is 
not a very bad conductor. The surface resistance is in that case 
too great to give good results. I intend to make further experi- 
ments on the conduction along the axis of quartz, which, along 
with a continuation of this investigation, I hope to have the 
honour of laying before the Society at a future time. 
But these remarks do not apply to really bad conductors, and I 
have every reason to believe that the numbers given above do not 
differ widely from the truth. 
