206 Dr-Bretvster on the Optical Structure of Lithion-Mka. 



remarkable fact, that these plates of Lithion-Mka were com- 

 posed of crystals with one axis, united to crystals with two axes, 

 and without the appearance of any joint or face of composi- 

 tion. By insulating the uniaxal portions, which occupied 

 much less space than the biaxal ones, I found that the charac- 

 ter of their axis was negative ; and by insulating the biaxal 

 portions, I found that the inclination of the resultant axes, af- 

 ter refraction, was almost exactly 45°, the principal axis being 

 also negative. The inclination of the axes before refraction 

 was 70°. In some other parts of the plate, which where irre- 

 gularly crystallised, I found the angle so high as 74° and 75°. 



Now, as all the uniaxal crystals of Mica that have yet 

 been analysed, differ from the biaxal ones in chemical composi- 

 tion, we would recommend it to Professor Gmelin to detach, 

 if possible, all the uniaxal parts from the biaxal parts, and to 

 make a separate analysis of both. If he shall find, what ana- 

 logy authorizes us to expect, that iliese two portions are che- 

 mically different, the result will be a most important one, both 

 for mineralogy and for analytical chemistry. It will set aside 

 all analyses of minerals, where it is likely that the body ana- 

 lysed has not been an individual crystal, and it may thus es- 

 tablish, upon a firmer basis, the law of definite proportions. 



In examining the Lithion-Mica with a microscope, I no- 

 ticed, in various places, considerable portions of a substance 

 lying between the laminae, which was of a bright scarlet co- 

 lour, whether seen by reflected or transmitted light. It is 

 now indurated, but seems, from its outline, to have been once 

 fluid. The origin and nature of this substance deserve to be 

 investigated. 



Art. IV. — Description of a Boat with a Revolving Paddle 

 Sndl, Invented by Andrew Waddell, Esq. F. R. S. E. 

 Communicated by the Author. 



The following is a sketch of a Boat with a revolving Paddle 

 Scull, of which an experiment was made in May 1824, on the 

 Inner Basin of the Wet Docks of Leith, by Mr Waddell of 

 Hermitage Hill. 



The boat was twenty-six feet long, and six feet broad, and 

 was propelled by means of the revolving scull projecting from 



