Mr. Nixon o?j the Inslrumeiilal Error oJJiis Horizon-Sector. 337 



duum is no longer soluble even in concentrated acetic acid. 

 Acetate of potash does not precipitate the salts of vanadium. 

 Concentrated acetic acid dissolves hydrate of vanadium, but 

 the solution becomes green by spontaneous evaporation, and 

 leaves a granular powder, composed of deep green opake 

 microscopic crystals, the form of which is either a cube or a 

 very short rectangular prism; they dissolve very slowly in 

 water; the solution is of a deep green colour. 



Formiate of Fanadium. — Artificial formic acid dissolves 

 hydrate of vanadium, and gives by spontaneous evaporation 

 a blue, opake, saline mass, which is easily soluble in cold 

 water ; but this solution, which contains no excess of water, 

 becomes gradually green by contact with the air. The salt 

 when perfectly dried is of a violet colour, with a tint of brown, 

 and is not completely soluble in water. 



Succinate of Vafiadium. — Succinic acid dissolves very little 

 hydrate of vanadium ; the solution is but slightly coloured. 

 By evaporation the succinate is obtained in the state of a 

 white powder, mixed with crystals of succinic acid. Neverthe- 

 less, the salts of vanadium are not precipitated by neutral suc- 

 cinates, but the mixture soon becomes greenish. 



Betizoate of Vanadium. — A boiling solution of benzoic acid 

 dissolves a little hydrate of vanadium; submitted to a slow 

 evaporation the solution deposits a yellow powder, and the 

 excess of acid crystallizes around it. 



[To be continued.] 



XLII. Particulars of the Measurement, by various Methods, 

 of the Instrumental Error cf the Horizo7i-Sector described in 

 Phil. Mag. vol. lix. Bij John Nixon, Esq. 

 [Continued from pege 96.] 

 By the Eleventh Method. 

 Theory. — T ET E (next page) represent the eye end, and O 

 -^-^ the object-glass end of the (horizontal line) of col- 

 jimation of a perfect telescopic-level (or of the horizon-sector) 

 correctly adjusted for taking a level. If we increase the diame- 

 ter of the cylindrical ring nearest the object-glass, by for in- 

 stance coiling a quantity of thread about it, the line of coUima- 

 tion will be elevated in the direction FP; and on reversing the 

 telescope within its Ys, it will be equally elevated in the op- 

 posite direction, or GQ*; the angle PEO = QOE being 



* Admitting the Ys to have the same angular opening; but it is evident 

 from tilt' demonstration given in pai;e 4»'!J of last volume, that were the 

 Ys unc'(jiial tlic theory would be eiiualiy applicable. 



h'.S. Vol. 10. No. 59. Nov. 1831. '2 X the 



