26 DR THOMSON'S ANALYSIS OF A MINERAL FROM THE TWEED. 



the mici oscope to be made visible ; and when placed under a high mag- 

 nifier is seen to be beautifully and finely reticulated, producing in the 

 sun's rays a play of colours. This ringed and reticulated appearance is 

 well seen in parts which have become dry. The skin is very thin, and, 

 when cut transversely, shews that internally the body is composed of a 

 white medullary substance, in the centre of which may be seen the intes- 

 tinal canal. The whole organization appears, as far as can be made out 

 or seen, to be very simple ; so simple, indeed, says honest Muller, that 

 " even by the aid of the microscope, nothing more fully can be made out 

 of it." The same author says farther, no appearance of a mouth can be 

 made out, though the very celebrated Plancus represents the mouth fim- 

 briated. Plancus' work I have not been able to see to compare his re- 

 presentation with what I have seen myself, and no author that I have 

 seen describes it as being visible ; but though all appearance of a mouth 

 escaped certainly for a time my utmost efforts, at length, by means of 

 the powerful microscope belonging to Sir John Hall, Bart, of Dunglass, 

 and after repeated examinations, I discovered at the very extremity in 

 several specimens, a round aperture, having somewhat of a fimbriated 

 margin round it. In other specimens in which the round aperture was 

 not visible, I could distinctly see the white medullary part of which the 

 internal part of the animal is composed, filling up the aperture, and in 

 many a puckering was distinctly seen, as if the mouth were closed. 

 Once, and once only, I distinctly, as I thought, saw the puckered ap- 

 pearance give way while under the microscope, and the round aperture 

 open to its full extent. This opening, therefore, I have no doubt is the 

 mouth of the animal ; and as upon repeated examinations no other open- 

 ing could be seen in any other part of the body, I conclude that this, as 

 in some other animals, may serve the purposes both of mouth and anus, 

 or that this latter aperture remains to be discovered. 



Analysis of a Mineral from the Tweed. By Dr THOMSON. (Read 

 December 21. 1831.) 



THE mineral of which the analysis is subjoined, occurs on the banks 

 of the Tweed, near St Boswell's, in connexion probably with the sand- 

 stone of that district. It is extensively used as a slate pencil in the 

 neighbourhood where it is found, and resembles indurated claystone. 



Colour milk white : opaque : lustre dull : sectile : hardness 2.5 : spe- 

 cific gravity 2.558. Before the blowpipe per se becomes blue and 

 brittle. Fuses with carbonate of soda into an opaque bead ; with borax 

 and salt of phosphorus into a transparent glass. 



Its constituents are 



