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 {tate of the marine acid, feem to be the 
caufe of this fcarce credible effet. ~ 
Ns Corrosive fublimate mercury, added to a folution of crude 
al ammoniacum in water, poffeffes the next place to the butter ~ 
of antimony among the difperfive fluids which I examined. 
It may be made of fuch a degree of flrength, 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 manifeftly 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. 
Tue 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 refraéting angle of a prifm of thefe, to the refracting an- 
gle of a prifm of crown-glafs ating in oppofition, is about 
two tothree. 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 reétified fpirits, and with nitrous and vi- 
triolic ether. 
A 
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