100 



through the gurn in the form of the point of a flattened cone, which is 

 coated with enamel, and downwards towards the jaw, increasing in 

 breadth, but not in thickness, till it is imbedded in the substance of 

 the jaw itself. The lower portion has no enamel ; the number of 

 teeth is, as described by Dr. Roxburgh, 120. 



Description of an Acid Principle prepared from the Lithic or Uric 

 Acid. By William Prout, M.D. Communicated by W. H. Wol- 

 laston, M.D. F.R.S. Read June 11, 1818. [Phil Trans. 1818, 

 p. 420.] 



The object of this paper is to show that the purple substance ob- 

 tained by heating a mixture of the lithic and nitric acids, is a com- 

 pound of ammonia with a peculiar acid principle, which the author 

 proposes to call Purpuric Acid, a term suggested by its peculiar ten- 

 dency to form red or purple compounds. 



The purpuric acid is obtained by digesting pure lithic acid in di- 

 lute nitric acid, neutralizing the excess of the latter by ammonia, and 

 evaporating till granular crystals, consisting of purpurate of ammonia, 

 separate. The ammonia is removed by sulphuric or muriatic acid, 

 and the purpuric acid thus obtained in a free state. 



The author next points out the characters of this acid. It is very 

 sparingly soluble in water, and insoluble in alcohol and ether. In 

 the mineral acids, and in the alkalies, it readily dissolves. It is in- 

 soluble in dilute sulphuric, muriatic, phosphoric, oxalic, citric, and 

 tartaric acids. When heated it neither melts nor sublimes, but be- 

 comes purple, from the production of ammonia, and then burns gra- 

 dually without any particular odour. It unites with the metallic 

 oxides ; and when aided by heat, expels carbonic acid from the alka- 

 line carbonates. It does not unite with any other acid. Upon these 

 characters the author thinks that its properties, as an acid, are suffi- 

 ciently established. 



Dr. Prout then proceeds to describe its compounds with different 

 bases, which, with few exceptions, are of a purple or reddish colour : 

 he thinks that some of them might be used as pigments, or employed 

 in the art of dyeing. 



Astronomical Observations and Experiments, selected for the purpose 

 of ascertaining the relative Distances of Clusters of Stars, and of 

 investigating how far the Power of our Telescopes may be expected 

 to reach into Space, when directed to ambiguous Celestial Objects. 

 By Sir William Herschel, Knt. Guelp. LL.D. F.R.S. Read June 

 11, 1818. [Phil. Trans. 1818,;?. 429.] 



Having shown in a former paper that by an equalization of the 

 light of stars of different brightness, their relative distances from the 

 observer in the direction of the line in which they are seen may be 

 ascertained, and having deduced from this equalization a method of 

 turning the space penetrating power of a telescope into a gradually 



