IA2 HEMIHEDRISM TPIE TARTRATES. 



this class ot' liquids. The laevo-gyration of narcotiue, in alcohol and ether, is 

 151°. 4; that of sulphate of quiniue, in water acidiilatcd with sulphuric acid, is 

 192°.95 in the same direction. Solution of crystallizahle cane sugar is dextrogyre ; 

 that of uncrystallizable cane sugar, or molasses, is Irevogyre. Solution of sugar 

 of grapes is also dextrogyre when prepared from the juice, and before solitlifica- 

 tiou; hut if evaporated to dryness and redissolved, it is lajvogyre. Crystallizable 

 cane sugar is made imcrystallizable by heat, and its rotatory power is accordingly 

 reversed l^y the same cause. -In many solutions the introduction of an acid 

 modifies the rotatory power. Narcotiue, from being— 151°. 4, becomes, after the 

 addition of hydrochloric acid, 4-83^. Cane sugar has its rotatory power inverted 

 in the same way. Upon this principle is founded the construction of Soleil's 

 saccharimcter just mentioned. A solution of the sugar to be examined is made 

 of a definite density, and its rotatory power observed in a tube twenty centimetres 

 in length. There is then added to the solution a measured amount, oue-tenth 

 of its volume, of strong hydrochloric acid, and a heat of about 150° F., applied 

 for ten minutes ; after which it is cooled and observed again in a tube one-tenth 

 longer than before. Its rotation Avill now be wholly negative. The original 

 observation will give the difference between the rotatory effects of* the crystal- 

 lizable and uncrystallizable sugar present, and the second observation will give 

 the sum of the same effects. From these data the relative quantities of the 

 two kinds contained in the solution may be deduced. For convenience, tables 

 to accompany the instrument are prepared in advance, from which the values 

 sought may be foiind by inspection. A saccharimcter has also been conti'ived 

 by Mr. Mitscherlich. 



Mr. Pasteur has made a very elaborate examination of the salts of tartaric 

 and paratartaric acid in their relations to polarized light. All the tartrates are 

 dextrogyre ; the paratartrates have no rotatory power at all. Mr. Pasteur made 

 the interesting discovery that paratartaric acid which is the same as racemic, and 

 which differs from tartaric acid only in having an additional atom of water, is 

 composed of two acids, one of which has a positive and the other a negative rota- 

 tory power. The dextro-racemic acid is simply tartaric acid, and the dextro-race- 

 mates arc tartrates. Paratartaric acid and its salts owe their neutral character 

 to the balance of opposite forces belonging to their components. 



In considering the crystalline forms of these different salts, Mr. Pasteur de- 

 tected a relation between them and their polarizing properties, such as has already 

 been described to exist in quartz ; that is to say, the salts which possess rota- 

 tory power have plagihedral faces leaning in the direction of rotation. The 

 crystals are all of the kind called by Mr. Weiss hemihedral ; that is to say, not 

 in all respects symmetrical. Mr. Pasteur observed that there are two kinds 

 of hemihedral ciystals, which he has distinguished as the stipcrposahlc and the 

 non-supcrj)Osahlc. When a crystal, or any solid, or surface is such that another 

 may be conceived or constructed like it in every particular as to form and dimen- 

 sions, yet incapable of being made to occupy the same matrix or mould, such a 

 crystal, or solid, or surface belongs to the class of the non-super posahlc. The 

 image of the face in a mirror, as compared with the face itself; the left hand or 

 the left foot, as compared with the right, and many analogous objects natural and 

 artificial, may serve to illustrate this conception. Mr. Pasteur found that all the 

 crystals whose salts possess the rotatory power are hemihedral and non-super- 

 posable; and, conversely, that all salts whose crystals are non-superposably 

 hemihedral have the power of circular polarization, with two exceptions only 

 thus far known, which are formiate of strontian and sulphate of magnesia. In 

 the latter case the crystal is so very nearly superposable, that it is hardly sur- 

 prising that it should not sensibly conform to the law. In the instance of the 

 formiate of strontian, Mr. Pasteur thinks that the hemihedrism does not depend 

 on the arrangement of atoms in the chemical molecule but on that of the physi- 

 cal molecules in the entire crystal ; so that, on solution, the structure on which 



