METHOD OF EXAMINING DISPERSIVE POWERS. 97 



It may here be feen, that feveral of the metals increafe the Gold and platina 

 difperfive powers of nitric and muriatic acids, and confequently "™ ft "«reafe the 

 exceed them in that refpect. Of all thefe fubfiances that Ii ea ft. 

 have yet tried, gold and platina are the moft difperfive. The 

 leaft difperfive of the metals is zinc. 



The earths alfo are found to pofTefs this property in very The earths differ 

 different degrees : that of the jargon and magnefia differ but much * 

 little from nitric acid in difperfive power; but iiliceous earth, 

 on the contrary, is inferior to water. 



By comparing the falts formed with the nitric and muriatic Nitric acid dif- 

 acids, it appeared probable that the former had the -higher dif-^^^ " than 

 perfive power ; but a more direct comparifon could not be made 

 by means of the rectangular piece of plate-glaf% as muriatic 

 acid could not be rendered fufficiently denfe for fuch a trial ; I 

 therefore made ufe of a triangular prifm of crown-glafs, which 

 is in itfelf lefs difperfive than any plate-glafs, and, from the 

 relative pofition of its furfaces, occationed lefs correction of the 

 colours. With this prifm, I found that ftrong muriatic acid 

 (having a refractive power 1,394) exhibited the colours re- 

 verfed ; 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 difperfive power of nitric acid, when tried 

 by the fame prifm, 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 deitroyed. 



In the table it may be obferved, that the red and green mu- More oxygen 

 riates of iron, though confifting of the fame metal aqd acid, ^M^to" 

 differ very much in difperfive power ; and, confequently, thatin the muriate 

 fbme caution will be neceffary, in attempting to compare the ofiron » 

 different metals with each other by means of the falts contain- 

 ing them, as any difference obferved 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 ftriking inftance of the latter is manifeft, from a compa-but lefs in the 

 rifon of fulphur with the fulphuric acid; for, while the former J* lc *ationof 

 appears to exceed the metallic oxides in difperfive power, the 

 latter is inferior even to water. 



As I have likewife, at various times, made many experi- The order of 

 ments on difperfion by means of wedges, in a manner nearly vari ° u . 3 ****** a9 

 fimilar to that employed by Mr, Dollond, Dr. Blair, and others, perfive power* j 



Vol. IV. — February. H I have 



