234 On Cavities in Amber containing Gases and Fluids. 



In my second paper " On the Refractive Power of the two new 

 Fluids in Minerals," I have mentioned some specimens of amber, 

 shown me by the late Mr. William Nicol, in which the inner 

 surfaces of some of the cavities were rough, like finely-ground 

 glass, and contained a fluid with a moveable vacuity*. Upon 

 subsequently examining one of these specimens, I found that the 

 apparent roughness of the cavity arose from its surface being 

 covered with minute strise parallel to one another. Some of the 

 cavities in the specimen were very irregular, while others were 

 perfectly spherical ; the latter were quite full of fluid, a fact which 

 became obvious by comparing the refraction at their boundaries 

 with the refraction at the boundaries of the other cavities which 

 had a vacuity in the fluid they contained. Mr. Nicol was of 

 opinion that the vacuity had increased in several of them, a result 

 which could arise only from its absorption by the amber. 



In a beautiful specimen of amber in the collection of the late 

 Mr. Witham of Larkington, which I had occasion to examine, 

 there were about eight spherical cavities like balls, all very close 

 to each other ; and some of them so nearly in contact, that they 

 were separated by a very thin film of amber. The fluid which they 

 contained was a heterogeneous dark yellowish-brown substance. 

 I opened one of the cavities in the presence of the late Dr. Edward 

 Turner and Mr. Witham, and took out the fluid, which, to all of 

 us, had the smell of soot. When placed upon a plate of glass it 

 could be drawn along like the white of an egg, or the vitreous 

 humour of the eye. When dried, it left a transparent matter 

 like amber. This matter had no double refraction. When ex- 

 posed to heat under the blowpipe, it became of a fine orange-red 

 colour. It did not burn, but by a continuance of the heat it 

 became black and disappeared. 



In a very fine specimen of amber, belonging to the late W. G. 

 Thomson, Esq., and of the size shown 

 in the annexed diagram, there was 

 a large oblong cavity ab, in which 

 a fluid with its surface concave stood 

 at the level mn. The fluid was so 

 sluggish, or probably viscid, that in 

 cold weather it required to be shaken 

 to make it move to the end A by in- 

 verting it ; but in warm weather 

 (64° Fahr.) it moved with great facility. 



The extremity b of the cavity was so 

 near the surface of the specimen, that 

 I durst not make any experiments 

 upon the fluid by heating it lest the 



♦ Edinburgh Transactions, vol. x. p. 424, note. 



