1885.] 



Underground Temperatures, fyc. 



161 



Having filled a fresh, tribe with fresh spun glass, I carefully 

 exhausted with the Sprengel pump on January 24th, and the 

 exhaustion was kept up till February 5th, that is, for twelve days. 

 During this time I frequently tested with the McLeod gauge. A very 

 slight increase of pressure was found during that interval ; but it was 

 so slight that I am not able to say that it was greater than that 

 which is observed at all times, even with the Sprengel pump in 

 excellent order, when a vacuum is maintained for several days. 



On February 5th, I passed three or four bottlesful of mercury 

 through the pump, and had a vacuum of about 0'5 M as shown by 

 the McLeod gauge. I then applied heat, and had instantly an abun- 

 dance of gas given off from the spun glass. This was collected as 

 before, and analysed. 



The number of glass fibres was 15,500, giving an estimated surface 

 area of 3527 sq. centims. The amount of gas given off was 0*41 c.c. ; 

 which is considerably less in proportion than m my first experiment. 



Of this gas it was found that 78*6 per cent, was carbonic acid gas 

 (absorbable by caustic potash). Of the remainder 10' 5 per cent, was 

 oxygen (absorbed by pyro gallic acid and potash) ; while 89*5 per 

 cent, was left unabsorbed; and may be supposed to be mainly 

 nitrogen. 



The very large proportion of carbonic acid gas is remarkable, 

 and it is difficult to account for, unless we may suppose that it was 

 taken up by the glass in large quantity during the operations of 

 drawing out the glass into fibres, and enclosing it in the containing 

 tube — operations during which there was, in these preliminary experi- 

 ments, an abundant supply from the blowpipe flames. 



II. " On Underground Temperatures, with Observations on the 

 Conductivity of Rocks, on the Thermal Effects of Satura- 

 tion and Imbibition, and on a special Source of Heat in 

 Mountain Ranges." By Joseph Prestwich, M.A., F.R.S., 

 Professor of Geology in the University of Oxford. Received 

 January 24, 1885. 



(Abstract.) 



The author remarks on the difference of opinion between physi- 

 cists and geologists respecting the probable thickness of the outer 

 crust of the earth — the former, on the strength of its great rigidity 

 and the absence of tides, contending for a maximum thickness and 

 comparative solidity of the whole mass ; while the latter, in general, 

 on the evidence of volcanic action, the crumpling and folding of the 

 strata in mountain ranges, its general flexibility down to the most 



