410 Prof. A. De la Rive's Researches on the Magnetic 



magneto-rotatory powers at temperatures differing by 10° remains 

 sensibly the same, with a slight tendency to increase as the tem- 

 perature rises. But we do not find, as with those liquids on 

 which we operated at first, the ratios between the rotations equal 

 to those between the corresponding densities; the latter are much 

 less, especially at the lowest temperatures. Thus, in water, 

 the effect of rise of temperature upon the magneto- rotation does 

 not depend solely upon the diminution of the density ; it acts 

 independently, and the more in proportion as the absolute tem- 

 perature is lower. For example, the ratio between the density 

 at 40° and the density at 51° is 1*0048, while the ratio between 

 the densities at 10° and 21° (which, like the preceding, differ by 

 11°) is only 1*0017; and yet the ratio between the rotatory 

 powers is the same (1*008) in both cases; it should have been a 

 little less in the second than in the first. There must, then, 

 have been two factors in the influence of the temperature upon 

 the magneto-rotatory power — the one depending on the variation 

 of the density, the other on the temperature itself independently 

 of its influence on the density — both factors acting in the same 

 direction ; in other words, rise of temperature diminishes the 

 magneto-rotatory power both by diminishing the density and 

 directly. 



The following Table of observations upon monohydrated sul- 

 phuric acid contains the means of the results furnished by two 

 distinct series of experiments : — 



Acid (HO SO s ). 



20 C. 



O 1 



8 



30 „ 



7 52 



40 „ 



7 50 



50 „ 



7 43 



60 „ 



7 35 



Temperatures. 



Ratio between Ratio between 



o o 



the densities. the rotations. 



20 and 30 



1*0059 1*010 



30 „ 40 



1-0059 1-010 



40 „ 50 



1-0058 1-015 



50 „ 60 



1-0055 1-017 



20 „ 60 



1*0231 1-055 



Thus with sulphuric acid, as with water, the magneto-rotatory 

 power diminishes with the rise of temperature more rapidly than 

 it would if the diminution were dependent only on the density. 



It therefore appears to me evident that, although diminution 

 of density may be the principal cause of the effect in question, it 

 is not sufficient in all cases to explain it, and that the molecular 



