92 Prof. J. D. Forbes on an Instrument for measuring 



The unaltered rock is a dolomite containing nearly 70 pef 

 cent, of carbonate of lime, and nearly 20 per cent, of carbonate 

 of magnesia ; while the altered rock contains but from 1 to 2 

 per cent, of the latter ingredient. To what cause are we to 

 assign the changes that have taken place ? Has the magnesia 

 been sublimed by heat? or has it been withdrawn by the sol- 

 vent power of free carbonic acid ? On the nature of these 

 and the other chemical changes that have been induced, I do 

 not feel myself competent to express an opinion ; and from 

 such limited premises it would be unphilosophical to draw 

 any general conclusions. The subject is one, however, of 

 great interest both to the geologist and chemist, as the facts 

 are directly opposed to the received views* ; and as no instance 

 of similar changes on dolomitic rocks has, so far as I am aware, 

 ever been put on record. 



High School of Glasgow, 

 June 1849. 



XII. Notices. By Prof. J. D. Forbes f. 



1, On an Instrument for measuring the Extensibility of Elastic 



Solids. 



THIS instrument is almost a faithful reproduction of 

 S'Gravesande's apparatus described in his Physices Ele- 

 menta Mathematical 1742 (but not in the previous editions). 

 It is described or alluded to by few modern writers, except 

 Biot in his Traite de Physique. It consists of a strong 

 wooden table or frame, with a vice at each end, between which 

 a wire or lamina may be stretched with a determinate tension 

 by means of a weight attached by a cord, passing over a pulley 

 in the manner of the musical apparatus called a monochord. 

 After the tension is adjusted both vices are screwed fast, the 

 space included between them being exactly fifty inches. If, 

 now, any deviation of the middle point of the wire included by 

 the vices be made (similar to the action of sounding a harp- 

 string), the force required to pull it a certain distance aside 

 will depend, — 1st, on the length of the wire; 2nd, on its ten- 

 sion; 3rd, on its extensibility, or the modulus of elasticity. 



S'Gravesande employed his apparatus to verify Hooke's 

 law, that the extension is as the extending force within the 



* For the latest and best account of the views of geologists on this sub- 

 ject see the well-known work of Dr, Daubeny on Volcanos, 2ncl edit., re- 

 cently published ; also an admirable resume in the eloquent address of 

 Prof. Oldham of Dublin University, which has just appeared. 



t From the Proceedings of the Royal Society of Edinburgh. 



