Mitscherlich, one of the greatest chemists of the time, had just announced that certain salts--the tartrates and paratartrates of soda and ammonia--"had the same chemical composition, the same crystalline form, the same angles in the crystalline condition, the same specific weight, the same double refraction and, consequently, the same inclination of the optic axes. Notwithstanding all these points of similarity, if the tartrate is dissolved in water it causes the plane of polarized light to rotate while the paratartrate exerts no such action." Pasteur could not believe that all the chemical and physical qualities of two substances could be identical and their action to polarized light be so different. Mitscherlich was known, however, as an extremely careful observer. For several years Pasteur revolved all the possibilities in Mitscherlich's observations and, finally, came to the conclusion that there perhaps existed in the paratartrates, as prepared by Mitscherlich, two different groups of crystals, the members of one of which turned the plane of polarization to the right, the other to the left. These two effects neutralized each other and apparently the paratartrates have no influence on the polarized beam of light. Pasteur found that the paratartrates were composed of crystals that were dissymmetrical--that is, whose image reflected in a mirror cannot be superposed on the crystal itself. This idea Pasteur makes clear by reference to the mirrored image of a hand. The image of the right hand as {298} seen in a mirror is a left hand. It cannot be superposed on the hand of which it is the reflection any more than the left hand can be superposed on the right and have corresponding parts occupy corresponding places. Pasteur found that the paratartrates were not only dissymmetrical, but that they possessed two forms of dissymmetry. The mirrored image of some of the crystals could be superposed on certain of the other crystals just as the mirror image of the right hand can be superposed on the actual left hand. He concluded that if he separated these two groups from each other he would have two very different substances, and so the mystery propounded by Mitscherlich would be solved.