396 Scientific Intelligence — Arts. 



beriza rutila^ Grus antigonCy and Fulica pullata. — (Comptes Ren- 

 dus, tome xx., p. 1353.) 



41. Boring Power of Land- Snails on Limestone. — Few persons 

 are, I believe, aware of the fact, which I alluded to at the meeting 

 of the British Association in Cambridge last year, on the occasion of 

 a notice by Dr Buckland, " On the agency of land-snails in forming 

 holes and trackways in compact limestone," that this phenomenon 

 had been noticed many years ago by the late amiable and talented 

 author of the History of Northumberland, the Rev. John Hodgson, 

 on accurate observer of nature, who, in 1827, pubhshed in that 

 work (Part 2, Vol. i., p. 193) the following passage ; — " On a sunny 

 bank near Whelpington, a stratum of limestone" (carboniferous) " is 

 here and there seen in grey projecting masses, the under surface of 

 which is bored upwards into cylindrical holes, which are from a line 

 to four inches deep, and tenanted, especially in winter, by the banded 

 and yellow varieties of the Helix nemoralis. The Umax, while it 

 occupies these cavities during the summer, has its fleshy longitudi- 

 nal disk protruded out of the shell, and coiled nearly into a circle 

 on the surface of the stone, the summit of its shell hanging down- 

 wards ; and in this position it probably elaborates its den in the same 

 manner that some of the pholades work their way into clay and wood, 

 or, by a slow but constant process, sink and enlarge their cells in 

 the hardest stones.*' I had, some time previously to the date of this 

 publication, examined the spot, and was satisfied with the correct- 

 ness of Mr Hodgson's observations, and last October (1845) took 

 advantage of an opportunity to revisit it, and was confirmed in the 

 opinion I had before formed on the subject, and in the perfect accu- 

 racy of the description quoted above. 



The thoroughly sheltered position of the under surface of the rock 

 •precludes the possibility of the holes being an effect of weathering ; 

 and I feel convinced that they are the result of the slow, but nearly 

 constant action, of a weak acid secreted by the snails, which instinc- 

 tively, for the sake of shelter, would resort to such a situation, and 

 thus, in the course of ages, such holes would be formed in any sub- 

 stances on which the acid could act. — Communicated by W. C. Tre- 

 velyan. 



ARTS. 



42. Iridescent Silver. — (In a note from Prof. John Brockelsby of 

 Trinity College, Hartford, Conn., to B. Silliman junior). It is well 

 known to those who are conversant with optical phenomena, that the 

 brilliant play of prismatic colours exhibited by mother-of-pearl is 

 due to the structure of the surface, provided the shell is cut and 

 polished in a particular manner. This interesting fact was an- 

 nounced to the scientific world in 1829 by the discoverer, Dr 

 Brewster, who successfully transferred, by pressure, the splendid 



