Geological Society. 



155 



This proposition results not from any a priori deduction, but is 

 verified by the whole of facts known to this day. 



7. One may ask if the change of volume, in which the gases keep 

 the whole heat produced by their mutual actions, is regulated by a 

 simple law, analogous to those that have been observed when the 

 gaseous combinations are brought to the same temperature ; never- 

 theless it does not appear to be so. 



Let us compare the formation of the different hydracids by means 

 of their gaseous elements, which gives no change of volume when 

 the gas is reduced at 0° and m 760. 



The formation of chlorhydric gas, H CI, produces 23,900 calories ; 

 the formation of bromhydric gas, H Br, produces 13,400 calories ; 

 the formation of iodhydric gas, HI, produces 800 calories. The 

 specific heat of these gases being nearly the same under the same 

 volume, it is clear that the quantities of heat aforesaid cannot pro- 

 duce an augmentation of volumes identical or proportional with simple 

 numbers. 



GEOLOGICAL SOCIETY. 



[Continued from p. 79-] 



February 22, 1871. — Joseph Prestwich, Esq., P.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. "On supposed Borings of Lithodomous Mollusca." By Sir W. 

 C. Trevelyan, Bart., M.A., F.G.S. 



2. " On the probable Cause, Date, and Duration of the Glacial 

 Epoch of Geology." By Lieut.-Col. Drayson, R.A., F.R.A.S. 



In this paper the author started from the fact that the pole of the 

 ecliptic could not be the centre of polar motion, as the pole varied 

 its distance from that centre. He indicated the curve which the 

 pole did trace, and this curve was such as to give for the date 

 13,000 B.C. a climate very cold in winter, and very hot in summer 

 for each hemisphere. The duration of the glacial epoch he fixed 

 at about 16,000 years. The calculations resulting from this move- 

 ment were stated to agree accurately with observation. 



3. " On Allophane and an allied Mineral found at Northampton." 

 By W. D. Herman, Esq. 



In this paper the author gave analyses of an amorphous, trans- 

 lucent, reddish-yellow mineral, found incrusting sandstone in the 

 Ironstones of the Northampton sands, the comparison of which with 

 Mr. Northcote's analysis of allophane from Charlton leads him to 

 infer the identity of the two minerals. He also noticed a soft 

 white substance found in certain joints in a section of the North- 

 ampton sand, and also referred to allophane by the late Dr. Berrell, 

 who analyzed it. This substance was said to occur not unfrequently 

 in the Inferior Oolite of the Midland Counties. By analysis, it was 

 shown to agree nearly with Sanioite and Halloysite. 



