218 Mr. Nixon on Rater's Horizontal Floating Collimator 



s 



Minute quantities of foreign liquids soluble in water like- 

 wise disposed the saline solution to immediate crystallization, 

 as might be expected, and none with greater effect than al- 

 cohol. It is known that alcohol can precipitate sulphate of 

 soda from its aqueous solutions. The soluble gases I suppose 

 to possess a similar property. 



These facts appear to warrant the conclusion, that air de- 

 termines the crystallization of supersaturated saline solutions, 

 by dissolving in the water, and thereby giving a shock to the 

 feeble power by which the excess of salt is held in solution. 



%* Since the foregoing observations were printed, the 

 author has perceived that M. Gay-Lussac, in his paper on 

 crystallization, (Ann. de Chim. torn, lxxxvii.) had distinctly 

 thrown out the same theory as a conjecture, although the cir- 

 cumstance is not noticed by any systematic chemical writer., 

 But as M. Gay-Lussac brings forward no experimental illus- 

 tration of the theory, and indeed adduces one experiment as 

 unfavourable to it, the experimental confirmation of the theory 

 is novel, and was certainly required. 



XXXVII. Method of avoiding certain sources of inaccuracy 

 in the use of Kater's Horizontal Floating Collimator. By 

 J. Nixon, Esq. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 TN making use of the horizontal floating collimator of Capt. 

 ■*- Kater, in order to determine the error of collimation of the 

 telescope of a mural circle, it is necessary to place the colli- 

 mator first to the north and afterwards to the south of the 

 circle. In addition to the consequent probable source of 

 error, as pointed out by Capt. Kater, may we not enumerate 

 the following? 



1. In passing from the north, through the zenith to the 

 south, the telescope describes an arc, which, from its magni- 

 tude, may give rise to a sensible error in the graduation. 

 2. The telescope, unless quite uniform in its parts, may have 

 its flexure varied in consequence of being inverted in position ; 

 in which event, the error of collimation will not be the same 

 with the telescope pointed to the north as when directed to- 

 wards the south. 3. When the north and south sides of the 

 observatory are not uniform in temperature, is it not possible 

 that the difference may vitiate the observations ? 



All these sources of inaccuracy and doubt may be avoided, 

 and the observations completed with the telescope of the cir- 

 cle 



