UPON THE DIFFERENTLY COLOURED RAYS OF LIGHT. 21 



87. Water. 



Water acts more powerfully upon the green rays than flint 

 and crozm glass, rock-crystal, the white of an egg, and the great- 

 er number of the acids. It acts less powerfully, however, than 

 the phosphorous, sulphurous, phosphoric and sulphuric acids. 



88. Wormwood, Oil of. 



This oil exceeds flint and crown glass in its action upon green 

 light, but is inferior to oil of sassafras and oil of cloves. 



89. Zircon. 



This mineral acts less powerfully upon green light than 

 crown glass, but no secondary spectrum was visible when it re- 

 fracted in opposition to flint-glass. 



In order to present under a general view the results of the 

 preceding experiments, I have arranged the various substances 

 in a Table, inversely according to their action upon green 

 light ; that is, the bodies at the top of the Table form spectra 

 in which the red and green spaces are most contracted, and the 

 blue and violet ones most expanded. The relative position of 

 several of the substances, particularly the essential oils, is quite 

 empirical ; but it can readily be found by a reference to the 

 preceding experiments, whether or not the relative action of 

 any two bodies has been actually determined. To have 

 attempted to fix the exact place of every substance, would have 

 required a great degree of labour, and more time than I could 

 easily command ; and it is almost certain, that the Table 

 would still have been incomplete, as there are many of the 

 essential oils, and some minerals, which do not exhibit 

 any difference in their action upon green light. Several 

 of the oils, too, which I used, had been exhausted by re- 

 peated 



