29a Mr HerschePs Table of the 



different oils, acids, &c. though no doubt accurately determin- 

 ed from the particular specimens under examination, are yet, 

 as scientific data, deprived of most of their interest from the im- 

 possibility of stating precisely what was the substance exa- 

 mined. Most of the fixed oils are probably (as appears from 

 the researches of Chevreul) compounds in very variable pro- 

 portions of two distinct substances, a solid concrete matter 

 (stearine,) and a liquid (elaine,) and it is presumable that no 

 two specimens of the same oil agree in the proportions. This 

 is probably, peculiarly the case with the oil of anise-seed*, 

 which congeals almost entirely with a very moderate degree 

 of cold. An accurate re-examination of the refractive and dis- 

 persive powers of natural bodies, of strictly determinate chemi- 

 cal composition, and identifiable nature, though doubtless a 

 task of great labour and extent, would be a most valuable 

 present to optical science." 



At the end of the table of dispersive powers, which Mr 

 Herschel gives from Dr Brewster's Treatise on New Philosophy 

 cal instruments, he adds " respecting the results in this table, 

 the remark applied to that of refractive indices may yet be 

 more strongly urged. The whole stands in need of a radical 

 reinvestigation.'" 



Although we entirely agree with Mr Herscliel in all these 

 remarks, yet, as they are particularly applicable to our ob- 

 servations, which form the greater part of the tables referred 

 to, we think it necessary to add the following explanations. 

 • In order to obtain uniform measures of the refractive pow- 

 ers of oils and other fluids, it is absolutely necessary to deter- 

 mine their specific gravities, and the temperatures at which 

 the observations are made. But even if we do this, we shall 

 find that the same oil, especially if it is procured in small 

 quantities, will give different results, even at the same tempe- 

 rature. The more volatile parts fly off, and the oil becomes 

 inspissated, and has a higher refractive power. We have now 



• As a proof of the correctness of this remark, I may observe, that the 



oil of anjse-seed, whose refractive and dispersive power I measured, and 



which I have used in various optical inquiries, never congealed. That 



which I am using now, and which I consider pure, congeals entirely at 



60° Fahr.— D. B. 



4 



