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14 On the UNEQUAL 



fubftance, of an equal refracting angle, oppofed to them. The 

 great quantity of the femi-metal retained in folution, and the 

 highly concentrated ftate of the marine acid, feem to be the 

 caufe of this fcarce credible effect. 



Corrosive fublimate mercury, added to a folution of crude 

 fal ammoniacum in water, pofTeffes the next place to the butter 

 of antimony among the difperfive fluids which I examined. 

 It may be made of fuch a degree of ftrength, as to require a 

 wedge of crown-glafs, of double the refracting angle, to re- 

 move the colour which a prifm of it produces. The mercury 

 and marine acid contained in this folution, are manifeflly the 

 caufe of its difperfive power. For neither the water nor the 

 volatile alkali, which are its other component parts, will be 

 found capable, if tried feparately, of contributing towards this 

 effect. 



The effential oils were found to hold the next rank to me- 

 tallic folutions, among fluids which poffefs the difperfive qua- 

 lity. The moft difperfive I found to be thofe obtained from 

 bituminous minerals, fuch as the native petrolea, pit-coal and 

 amber. When the refraction is without colour, the proportion 

 of the refracting angle of a prifm of thefe, to the refracting an- 

 gle of a prifm of crown-glafs acting in oppofition, is about 

 two to three. The difperfive power of the effential oil of faffa- 

 fras, is not much inferior to thefe. The effential oil of le- 

 mons, when genuine, requires the refracting angles of the 

 prifms neceffary to produce a colourlefs refraction, to be as 

 three to four. In oil of turpentine, this proportion is as feven 

 to fix ; and the effential oil of rofemary is ftill lefs difperfive. 



Some expreffed oils which were examined, were found not 

 to differ fenfibly in difperfive power from crown-glafs, which 

 was alfo the cafe with rectified fpirits, and with nitrous and vi- 

 triolic aether. 



A 



