. 
On the Metallic. Sails, 33 
Thus I have endeavoured to show in what points this theorem 
is defective, not with any view to depreciate its value, but to assist 
in rendering it more perfect. It is the most simple one we have: 
and when the four unknown coefficients beforementioned, can be 
ascertained for all altitudes, densities and temperatures, a great 
obstacle to improvement will be removed, and the future pro- 
gress of astronomy much facilitated. 
Christ's Ilospital, Nov. 15, 1815. 
VILL. On the Meiéallic Salts. 
To Mr. Tilloch, 
Sir, — I BEG leave to obtrude myself again on your notice, to 
offer a few remarks on the metallic salts, in reply to the answer 
of your correspondent H. (p. 246, vol. xlvi,) to my paper (p. 44 
same volume). -This I fee! myself impelled to do, not only from 
the importance of the subject, but from some statements which 
Hi. has adduced as unobjectionable facts in support of the theory 
contained in his former paper. Before I proceed, however, it 
will be necessary to define the import of the words free and 
excess as hereafter used, philosophical discussions requirmg a 
strict adherence to terms. Indeed an omission of this kind has 
produced some degr ee of displeasure in your correspondent, who 
objects to my using the words free and excess indiscriminately, 
which alone should be attributed to the different opinions we 
entertain of the subject. By acid in excess I mean, when the 
superabundance of acid is chemically combined with the base, _ 
and forms a constituent part of the salt. By free acid, when 
inechanically united or only adhering to the salt. 
The inquiry is, if an excess of acid is really essential to the 
éxistence of all metallic salts. ‘The arguments on one side tend 
to show that it is essential, becausé their solutions constantly 
indicate acidity with the test ; on the other side it is maintained, 
that although free acid may, "and in general appears to be ne- 
cessary for solution, yet it is not required for the existence of 
the salt. 
As I endeavoured to prove that solubility was increased by an 
imerease of acid, H. in his answer draws the conclusion that, if 
such was the case, a salt would not be precipitated by acid in 
any proportion, and that the acid must be the solvent. Arid he 
alleges in opposition, that the greater: proportion of free acid, 
rather than render a metalic salt more soluble, will in most in- 
stances precipitate it, and that the water alone was the solvent; 
Vol. 47, No. 213. Jan. 1816. C otherwise, 
