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



much preferable to solids, on account of their dilatation being 

 equally uniform in all directions. In all those of which I have 

 studied a sufficient number of specimens, the magneto-rotatory 

 power diminishes with the rise of the temperature. Of this we 

 have already seen a striking example in the liquefied anhydrous 

 sulphurous acid ; for it is evident that, as already remarked, to 

 its great dilatability* are due the considerable variations of its 

 coefficient of magneto-rotatory polarization, which, from 1*272 at 

 4°, becomes 1*240 at 12°, and 1-207 at 20°. 



Essence of turpentine and creosote also present a sensible di- 

 minution of magneto-rotatory power in proportion as their tem- 

 perature is raised ; thus with essence of turpentine the rotation, 

 which is 10° at the temperature of 12° C, is only 9° at the tem- 

 perature of 80° C. 



With creosote the same' rotation is 19° at the temperature of 

 12° C, and only 17° 35' at the temperature of 80° C. Unfor- 

 tunately I was unable to make use of the experiments on essence 

 of turpentine and creosote to discover the ratio between the aug- 

 mentation of their volume and the diminution of their magneto- 

 rotatory power. For this purpose I should have had to make 

 direct determinations of their coefficients of dilatation — not being 

 able to depend upon those attributed to them, on account of the 

 differences of composition presented, as I have said, by the dif- 

 ferent samples of these two liquids submitted to experiment. I 

 have restricted myself to making this comparison with respect to 

 four liquids very different one from another, which I have been 

 able to procure very pure, and the coefficients of dilatation of 

 which have been very carefully determined by several physicists, 

 particularly M. Isidore Pierre. 



These four liquids are rectified alcohol (sp. gr. 0*804), iodide 

 of ethyle, water, and monohydrated sulphuric acid (HO SO 3 ). For 

 the first three, I made use of the Tables of dilatation given by 

 M. Isidore Pierre; and for sulphuric acid I used Muncke's 

 Table. 



The tube containing the liquid operated on was surrounded 

 by a metal case, the diameter of which was four times that of the 

 tube ; the case was filled with water ; and a spirit-lamp afforded 

 the means of gradually heating the water, a thermometer con- 

 stantly indicating its temperature. I took care, in each experi- 

 ment, to wait till the thermometer had become stationary, in 

 order to be sure that the interior liquid had exactly the tempe- 

 rature of the water j\ The precaution bad been taken of leaving 



* The coefficient of dilatability of liquid sulphurous acid is 0-0018 at 

 10° C, and 0*0036 at 80°— that is to say, nearly equal to that of air. 



t All the thermometric indications are in Centigrade degrees ; C. is put 

 after them in order to distinguish degrees of temperature from degrees of 

 rotation. 



