On the Influence of Temperature on the Refraction of Light. 223 



The following are the conclusions arrived at : — 



1 . In every substance the refractive index diminishes as the tem- 

 perature increases. This is seen in the first four columns of the 

 tabulated results, which represent the refractive indices of the fixed 

 lines of the spectrum A, D, and H respectively at the temperatures 

 indicated, while the succeeding column shows the amount of differ- 

 ence for each five degrees Centigrade. This change of refractive 

 index by heat, for which the term sensitiveness is proposed, varies 

 greatly in amount in different substances, melted phosphorus and 

 bisulphide of carbon being the most, and water the least sensitive of 

 the liquids examined. 



2. The length of the spectrum varies as the temperature increases. 

 The difference between the refractive indices of the lines A and H, 

 or jj.^ — /^A>"is taken as the measurable length of the spectrum, and is 

 given in the sixth column. In the case of highly dispersive sub- 

 stances, as bisulphide of carbon and hydrate of phenyle, it decreases 

 considerably ; in the case of less dispersive bodies, as the alcohols, it 

 decreases to a less extent ; while with water the change is not 

 appreciable. 



3. In some substances the dispersive power is diminished, in 

 others it is augmented by a rise of temperature ; that is, in such 

 substances as bisulphide of carbon, it is the numerator of the fraction 



— — - that decreases fastest, while in such substances as water it is 

 I'd-! 

 the denominator. The result of this is shown in the last column. 



4. The sensitiveness of a substance is independent of its specific 

 refractive or dispersive power. Thus water and ether are very 

 similar as to the actual amount of the refraction and dispersion 

 exhibited by them, but ether is many times more sensitive to heat 

 than water is. 



5. The amount of sensitiveness is not directly proportional to the 

 change of density produced by alterations of temperature ; yet there 

 is some relationship between the two phenomena. Thus in water 

 the index of refraction and the density both change much more 

 rapidly at high than at low temperatures ; again, the remarkable 

 reversion of the increase of density that takes place at 4° C. is not 

 without its indication in the amount of sensitiveness ; and the large 

 decrease of density at the freezing of water is accompanied by a 

 similar decrease of refraction. 



Moreover, as a general rule, those substances that are most affected 

 in density by heat are the most sensitive. 



G. No sudden change of sensitiveness occurs near the boiling- 

 point ; at least this is true in respect to bisulphide of carbon, etlicr, 

 atnd methylic alcohol. 



