ofl6 Dr. Wollaston's Method of examining 
The earths also are found to possess this property in very 
different degrees : that of the jargon and magnesia differ but 
little from nitric acid in dispersive power; but siliceous earth* 
on the contrary, is inferior to water. 
By comparing the salts formed with the nitric and muriatic- 
acids, it appeared probable that the former had the higher dis- 
persive power ; but a more direct comparison could not be made 
by means of the rectangular piece of plate-glass, as muriatic 
acid could not be rendered sufficiently dense for such a trial ; I 
therefore made use of a triangular prism of crown-glass, which 
is in itself less dispersive than any plate-glass, and, from the 
relative position of its surfaces, occasioned less correction of the 
colours. With this prism, I -found that strong muriatic acid 
(having a refractive power 1,394) exhibited the colours reversed; 
and that, when it was diluted till the limit of reflection appeared 
void of colour, its refractive power was reduced to 1,382. But 
the dispersive power of nitric acid, when tried by the same 
prism, proved to be greater ; for this acid required to be diluted 
till its refractive power did not exceed 1,375, before the colour 
was wholly destroyed. 
In the table it may be observed, that the red and green mu- 
riates of iron, though consisting of the same metal and acid, 
differ very much in dispersive power ; and, consequently, that 
some caution will be necessary, in attempting to compare the 
different metals with each other by means of the salts contain- 
ing them, as any difference observed may be owing in part to 
a difference in the quantity of acid to which they are united, 
and in part to their different proportion of oxygen. 
A striking instance of the latter is manifest, from a compa- 
rison of sulphur with the sulphuric acid ; for, while the former 
