96 



BRISTOL BUILDING STONES. 



arc derived." But before a loeal Society it will not, I hope, 

 be deemed unfitting to read a paper on local building stones. 

 I shall however make my remarks so far general as to 

 include such stones as are used in Bristol, though they be 

 brought from a distfiiioe. 



It is only right that, at the outset, I should acknowledge 

 my indebtedness (1) to my former pupil, Mr. V. W. Hard- 

 wick, who aided me in my experiments and in the detei'- 

 mination of the specific gravities and absorptive capacities * 

 of our local stones; and (2) to Messrs. W. Brock, John 

 Barstow, and otlicrs, for much valuable information most 

 courteously given. 



Of the three classes of building mateiials mentioned by 

 Prof. Ansted, one, the granite group, including the elvans, 

 porphyries, and traps, has not been extensively used in 

 Bristol. The widening of Bristol 15 ridge was executed with 

 Cornish gi'anito. Lundy granite was, I am told, used for the 

 harbour works at Cumberland Basin. I'or such, a pui'posono 

 stone could be bettor adapted than granite, since it absorbs but 

 little water. A cubic foot of the rock in the ordinary dry 

 condition (in which it already contains about one pint of 

 water, which can only bo liberated by artificial heat) is 

 capable of absorbing scarcely three-tenths of a pint more 

 on being placed in water. In a frost-bitten land, this small 

 absorptive power is essential in a building material for such 

 purposes. Granite is also a rock which, offers great resist- 

 ance to crushing stress, though difEorcnt classes vary con- 

 siderably in this respect. A cubic inch of Cornish granite 

 crushes under a load of 2'81 tons, while M.oi)nt Sorrel 



* On this subject see also Mr. E. Wethered's Paper on the Porosity 

 and Density of Bocks with regard to Water Supply [I'roc. Brist. Nat. 

 Soc). 



