C. GENERAL PHYSICS. 79 



POLARIZING ACTION OF TARTARIC ACID. 



In the extensive series of organic substances, there are some 

 that, as is well known, are endowed with the peculiar faculty 

 of deflecting the plane of polarization of the luminous rays. 

 This property was discovered by Biot, in 1815, in various liq- 

 uids among others, in spirits of turpentine and the laws 

 which most of these substances follow are, first, the rotation 

 produced by the liquids in the plane of polarization is propor- 

 tional to the length of the path which the luminous rays must 

 traverse in the liquid ; second, in the mixture of substances 

 endowed with the rotatory power with those that are inact- 

 ive, and which exercise no chemical action upon the former, 

 rotation is in proportion to the quantity of the active sub- 

 stance ; third, when several liquid columns are superposed in 

 the path of the luminous rays, the total rotation is equal to 

 the algebraic sum of the rotations peculiar to each of them; 

 fourth, the angle of rotation corresponding to the different 

 simple colors is very nearly in the inverse ratio to the square 

 of the length of the luminous rays. Tartaric acid does not 

 follow the law of Biot, constituting a special exception to the 

 second and fourth law. This anomaly induced Krecke to take 

 up the inquiry, the result of which he has lately published. 



The special points that he desired to investigate were, 

 whether the anomaly which tartaric acid exhibits at the or- 

 dinary temperature is seen also at a more elevated tempera- 

 ture; if the tartrates present the same anomalies as free tar- 

 taric acid ; and if tartrates follow the law of simple relations. 

 The results which he attained in the course of his inquiry he 

 sums up as follows : For all the rays of the spectrum the spe- 

 cific rotatory power augments with the temperature, but in a 

 quantity different for different solutions of the acid, and the 

 peculiar irregularity presented by tartaric acid namely, that 

 the green rays are displaced more than the yellow or the vi- 

 olet disappears with the augmentation of the temperature. 

 It decreases also in proportion to the increase of the quantity 

 of water, as had already been demonstrated by Biot. He 

 also informs us that the tartrates, as far as examined, follow 

 the laws of Biot ; that the molecular rotatory power is very 

 nearly the same in all the normal tartrates and alkaloids, but 

 considerably more in tartar-emetic; and that the molecular ro- 



