468 On t/ie Rotary Power which the Essence of Turpentine 



These verifications are not the only ones that we have made. 

 We have several times assured ourselves that the sum of the 

 intensities obtained for two rectangular positions of the analysing 

 spar is constant^ whatever these positions may be. 



Deviations. 

 riS^'o to the right of the zero 

 Principal section at < of the limb . . 



71°-5 to the left .... 7-2 



^"Hll-4 



T3 . . 1 ,. , r33°-5 to the rigl 

 Prmcipal section at ^g^c.g ^^ ^j^^ i^| 



Sum 18-6 



33°-5 to the risht of zero . 6-6 



. . . 11-7 ' 



Sum 1^3 ' 



The sumSj as we see^ differ very little. In these cases, the 

 principal sections of the two spars were parallel when that of 

 the analyser was about 20 degrees to the left of zero. ' » ' ti ► I 



All the precautions which we have indicated being taken, and 

 the analyser being fixed in such a position that the image which 

 fell upon the pile was extinguished, the caloi'ific action being at 

 the same time equal 0, a tube of glass, terminated by two parallel 

 glass plates and filled with an active liquid, the essence of tur- 

 pentine, for example, or a syi'up of siigar, was placed in the path 

 of the rays. The luminous image immediately reappeared, and 

 with it the thermoscopic action. But both of them could be 

 again rendered by a proper rotation of the analyser. 



In support of this assertion, we will cite some of the numbers 

 obtained by observing the rotation impressed upon the heat 

 which accompanies the green rays during their passage through 

 a tube 0'15 millim. in length, and filled with the essence of tur- 

 pentine. The plane of primitive polarization being vertical, and 

 the line 0° — 180° of the limb M'hich measured the rotation of 

 the analyser being also vertical, all calorific action was extin- 

 guished before the interposition of the active fluid when the 

 principal section of the analyser was at 90 degrees. 



After the interposition of the liquid, a deviation of 14°*5 was 

 obtained for the same })Osition of the prism, whereas when the 

 principal section was at the zero of the division, only a deviation 

 of 5*5 was obtained. The luminous image and the caloi'ific 

 action disappeared completely when the said section was at —32 

 degrees. 



These facts ])rovc incontestably, that the plane of polarization 

 of the calorific rays, when transmitted across an optically active 

 liquid, experiences a rotation of the same kind and in the same 

 sense as that experienced by the luminous rays. With regard 

 to the absolute magnitude of this rotation, some difficulty lay in 



