22 Report o/'Dr. Turner's Lecture 



remained solid, and on thawing left them to fall asunder by 

 the mere force of gravity. This was perhaps the most influ- 

 ential cause of the vast ruin daily witnessed in the valleys of 

 Switzerland, and in all countries where high mountain chains 

 are intersected by deep narrow gorges, bounded by bare, pre- 

 cipitous, and irregularly fissured escarpments. By the opera- 

 tion of the same cause buildings were defaced and destroyed. 

 When water froze within the cavities of porous stones, the 

 particles were frequently more or less disunited from each 

 other, and crumbled to dust at the first thaw. Building ma- 

 terials differed in their destructibility by frost. The compact 

 tenacious sandstone of Edinburgh suffered little ; while some 

 of the handsome colleges of Oxford gave melancholy proof of 

 the injury which it might occasion in the more porous and 

 less tenacious oolite of that county. The lecturer observed, 

 that a scientific knowledge of the cause of such decay had led 

 to the suggestion of a ready mode of estimating the durability, 

 as far as frost was concerned, of different building materials. 

 The freezing of water was a process of crystallization attended, 

 as in most other cases, with forcible increase of volume. The 

 crystallization of salts was a similar phenomenon, and gave 

 rise to a similar effect. When a stone was dipped into a sa- 

 line solution, and then suspended in the air to dry, the cry- 

 stallization of the salt produced a certain amount of injury; 

 and the effect due to one operation might be multiplied to any 

 extent by repetition of the same process. The experiment of 

 a few days might thus be made to imitate the effect of numer- 

 ous winters, and the relative durability of different materials be 

 ascertained prior to their selection for building. The salt most 

 applicable to such purposes was found to be sulphate of soda*. 

 3. Chemical Action. — The affinities which principally con- 

 tribute to affect the integrity of rocks were stated to be those 

 of water and carbonic acid for potash and soda, and that of 

 oxygen for iron. The changes referred to were frequent in 

 felspathic rocks, and were exemplified in a very striking man- 

 ner in the formation of porcelain clay from granite and other 

 allied rocks rich in felspar. All granitic regions presented ex- 

 amples of this nature, and in none were they more remarkable 

 than in Cornwall and Auvergne. It was probable that the 

 long-continued action of pure water might produce decompo- 

 sition; but the effect of its affinity for the alkalies of the rock 

 was materially aided by that of carbonic acid for the same 

 bases. This was shown by the increased decomposing power 

 of water when charged with carbonic acid, and by the action 



• M. Brard in Ann. de Chim. et de Phys., vol. xxxviii. p. 160. 



