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thought due to vegetation. The action of heat in drawing out- 
wards various crystallizing substances in the stone might keep 
parts clean that were exposed to the sun. 
Mr Bonney thought that the dampness or dryness of the 
stone, where sheltered from or exposed to the sun, was the 
chief cause; as favourable or unfavourable to both the growth 
of vegetation and the lodgment and chemical action of soot. 
Professor BABINGTON said that it was true there was much 
vegetation on the stone on the north side of King’s Chapel, but 
he attributed this blackness to smoke. 
(2) On Faye’s method of comparing métres a traits ; 
and an improvement of it suggested by Professor 
Mier, F.R.S. 
Diagrams of the instruments were exhibited and described, 
and a few remarks were afterwards made on them by Professor 
Maxwell. 
(3) On certain lithodomous burrows in the Carbonifer- 
ous limestone of Derbyshire. By T. G. Bonnzy, 
B.D. . 
The author referred to two previous communications on the 
same subject, and stated that some doubt having been expressed 
as to the accuracy of his observations in the most important 
case described in one of them, he had again visited the same 
neighbourhood. Not only had he confirmed his previous ob- 
servations, but he had found a large number of other burrows, 
which he described, exhibiting a very fine specimen; and he 
maintained these could not be (as had been said) the work of 
marine mollusca, as Pholades. It was very improbable that 
they would have lasted so long in limestone rocks; they were 
