On Metallic Salts. 463 



an intensity of light, this contrivance would afford a double ad- 

 vantage over coloured glasses : objects would be seen of thei? 

 true colour ; and the excess of light, which operates so power- 

 fully in impairing vision^ effectually corrected. ' 



1 am, &c, 



J. Skinner. 



LXXX. On Metallic Salts. By A CoRREspoNDE^!T. 



To Mr. Tilloek. 



Sir, — XJ.AVING continually observed that metallic solutions 

 had an excess of acid; although it appeared contrary to the 

 opinion of those whose merited reputation in the chemical 

 world is so firmly fixed, I was led to think that this acid was 

 essential to the salt; and the few experiments I made on the sub- 

 ject seem to justify my doubts. 



Sulphate (or rather the super-sulphate) of iron was the first salt 

 1 examined. It was submitted to a heat sufficient to drive off a 

 portion of the acid (as I knew without this process the solution 

 was acid) : water being added, the super-sulphate was in solution, 

 ftud some oxide precipitated. I endeavoured to saturate the 

 excess of acid by an alkali ; but before it could be neutralized a 

 part of the oxide precipitated, leaving the super-sulphate in so- 

 lution. If the acid was not a component part of the salt, it 

 might be saturated with a base, provided the salt produced would 

 tiot decompose the metallic solution : — if therefore it was not 

 essential, the alkali would saturate it, and the whole form a neu- 

 tral solution; for the dissolved salt is not subject to decomposi- 

 tion by treating it with an alkali saturated by the corresponding 

 acid to that wliich the metal is dissolved in. 



In the case of mercury, the salts termed the super-sulphate, 

 the sulphate, and the sub-sulphate, show acid properties when 

 dissolved: — if then the sulphate and the sub-sulphate in the at- 

 tempt at solution are converted into the super-sulphate, it is na- 

 tural to infer that they are not perfect salts, but peculiar mix- 

 tures of the super-sulphate with the oxide. — And as the excess 

 erf acid in the super-sulphate is held by so small an affinity, it is 

 not more entitled to be called an acid salt than those whose 

 bases possess a superior affinity for their acids, and are not like 

 this decomposed by water. Why I mention the coml)ination.s 

 of mercury with the sulphuric acid is, that in a publication pro- 

 duced in 1809, it is observed that, by repeatedly wasliing "the 

 super-sulphate with small quantities of cold water, the acid is 

 carried off, and " a truly ncutial metallic salt remain^, the siil- 



pliaLe 



