140 REPORT— 1878. 



mined characters capable of affording most useful assistance as permanent 

 and special qualities for distinguishing them from one another. 



Another example of comparison between the direct and indirect 

 methods of experiment has presented itself in the Committee's observa- 

 tions of Thermal Conductivities, where, however, they have not yet 

 investigated the original substance which was the subject of the earlier 

 experiments, but only an approximately similar one in two different con- 

 ditions which permit a fair comparison of the measurements to be made, 

 which have been obtained by direct, and also by a totally different method 

 of procedure. The quantity of water (23 per cent.) found to be taken 

 up by dry sand when thoroughly wet, as stated in the table, should raise 

 its specific heat by weight from 0*200, there given, to 0*348 ; but the 

 specific heat observed in the second case was only 0*284, corresponding 

 to an absorption of only 12 per cent, of water. The specific heats by 

 weight and by volume given in the table for thoroughly wet sand are, there- 

 fore, too low, and the value of the ratio - deduced from them must be 



c 



sensibly higher than its real value for saturated sand. For perfectly dry 



sand it is 0*0032, for thoroughly wet sand it must accordingly be about 



0-0100, and for the sand in the experimental garden in Edinburgh, Sir 



h 

 W. Thomson obtained the value 0*0087, of the ratio — , by a reduction 



of the records of Professor Forbes' underground thermometers. Could 

 a specimen of the sand itself, which the Committee hopes to procure, be 

 obtained in which the thermometers were sunk, the value of the ratio 

 found by Sir W. Thomson would no doubt be very closely corroborated ; 

 and, at the same time, the real values of the absolute conductivity and 

 specific heat of a loose and porous earth like that in which these ther- 

 mometers were placed, which the Committee has not yet determined, 

 would be added to the present list. 



The Committee has the satisfaction to notice with peculiar commenda- 

 tion the series of excellent and accurate experiments on the thermal con- 

 ductivities and capacities of certain specimens of rocks of Japan, which 

 the professors of the Tokio College there, Messrs. J. Perry and W". 

 Ayrton, have conducted with the greatest skill and originality of method 

 and ©f practical execution. Of the very excellent memoir of these 

 experimenters, and of the contributions from other sources to the practical 

 investigation of the subject of rock-conductivity which have been made 

 recently and in bygone years, the Committee hope to point out the 

 bearings in a collective review, if the production of such a historical 

 report, during the coming year, presents itself as a sufficiently desirable 

 object for their reappointment. Thin microscopic sections of about 

 twenty of the rock- specimens upon which they have experimented have 

 been prepared by Mr. G. F. Cuttell, of London, which convey much 

 information to the eye regarding the causes of the various degrees of con- 

 ductivity that are met with in particular rocks. The compact, almost 

 purely siliceous nature of the quartzites is thus visibly presented, and 

 the reason of their ranking with quartz much higher in conductivity 

 than the more heterogeneous sandstones is very obvious. A similar 

 minute inspection of the structure of Craigleith sandstone will no doubt 

 furnish evidence of similar purity of its material in comparison with other 

 sandstones, in explanation of the very distinctive quality of remarkably 

 high thermal conductivity which it appears to possess among them. 



