Astronmnical Methods of Observation. 45 



long, 4-^ inches wide, and 3 deep, so that it really deserves the 

 name I originally gave, that of a pocket azimuth and alti- 

 tude circle. With this little circle I can get, in one even- 

 ing, my latitude to within 5" of the truth by the pole star." 

 Such are literally the expressions of the late Captain Kater, 

 the inventor of this instrument, and the advantages of it 

 to scientific travellers are very obvious. 

 , {To be continued.) 



Article VIII. 



On a difficulty in Isomorphism, and in the received constitution 

 of the Oxygen Salts ; in a Letter to Professor Mitscherlich, 

 of Berlin, from Thomas Clark, M. D., Professor of 

 Chemistry in Marischal College, Aberdeen. 



{Concluded from vol. iii. page 443.) 



II. — Adopting the alternative of retaining the received 

 views of the constitution of the Oxymanganate of Barytes, 

 aud assimilating thereto our views of the constitution of 

 the Sulphate of Soda, consistency compels us to adopt the 

 following formulas : — 



Sulphate of Barytes, . . . Ba S 



Manganate of Barytes . . . Ba Mn 



Oxymanganate of Barytes, Ba Mn Mn 



Sulphate of Soda, . . . . So S S 



Manganate of Soda, . . . . So Mn Mn 



Oxymanganate of Soda, . . . So Mn Mn^ 



In this list, the alterations on the formulas are limited to 

 the salts of Soda. Those alterations have given rise to the 

 following difficulties : — 



1 . The basis of all the oxygen-salts of soda is assumed 

 to be an unknown oxide of sodium, retaining half as much 

 oxygen as is in soda. 



2. The sulphate of soda is assumed to contain, not sul- 

 phuric acid, but another undiscovered acid having in com- 

 bination additional oxygen, and by a like assumption, man- 



