1824.) Philosophical Tranactiom for 1823, Part IX. 299 



easy to produce an appearance of blue by using a prussiate, 

 which already contains iron, and is consequently better adapted 

 to prove the absence of iron where no blueness appears, than to 

 ascertain its presence, it is by no means easy to obtain the more 

 indisputable evidence of iron by infusion of palls. It is only by 

 repeated evaporation of the muriatic solution, and continued 

 exposure of the residuum to the temperature of boiling water, 

 that I have succeeded in separating enough of the titanium to 

 allow the blackness of gallate of iron to appear, when the efflo- 

 rescent edges of the dried salt are touched with infusion of galls. 

 " Although the quantity thus rendered sensible does not 

 appear in proportion sufficient to account for the magnetic force 

 observed, there seems more reason to ascribe it to this impurity, 

 than to suppose titanium possessed of that peculiar property in 

 a degree so far inferior to the other known magnetic metals." 



XXVII. An Account of the Effect of Mercurial Vapours on 

 the Crew of his Majesty's Ship Triumph, in the Year 1810. By 

 William Burnett, MD. one of the Medical Commissioners of the 

 Navy, formerly Physician and Inspector of Hospitals to the 

 Mediterranean Fleet. Communicated by Matthew Baillie, MD. 

 FRS. 



The curious facts to which this paper relates, having already 

 been detailed by Dr. Paris and Mr. Fonblanque, in their " Me- 

 dical Jurisprudence," vol. ii. p. 460, and the paper itself having 

 been reprinted in a contemporary journal, their repetition here 

 is unnecessary. 



XXVIII. On the Astronomical Refractions. By J. Ivory, 

 AM. FRS. 



We have endeavoured, in the following paragraphs, to convey 

 a general idea of the nature and contents of this voluminous and 

 elaborate communication, necessarily omitting the author's 

 mathematical details, but adopting, for the most part, his own 

 words. 



It was known to the ancient astronomers that there is a dif- 

 ference between the real and apparent places of the stars, arising 

 from the refraction of light in its passage through the atmo- 

 sphere. Tycho Brahe was the first who attempted to free his 

 observations from the effect of this irregularity. Since his 

 time the astronomical refraction has become more and more an 

 object of attention, as it is found to have the greatest influence 

 on the delicate exactness of modern observations. In the course 

 of the last twenty years many researches on this subject have 

 been published by philosophers of the first note, who have 

 applied all the resources both of theory and practice to over- 

 come the difficulties which it presents. By these means our 

 knowledge has been greatly extended ; but the problem of the 

 refractions must still be considered as the most imperfect part 

 of modern astronomy. 



The first investigation that presents itself in the problem of 



