The first care of Pasteur, after having discovered the left-handed tartaric acid and the constitution of paratartaric acid, was to compare very carefully the properties of the new left-handed acid with those of the right, endeavouring to determine by strict experiment the influence on these properties of the internal atomic arrangements of the two acids. Although we are unable to picture the exact figure of these atomic groupings, there can be no doubt that they are formed of the same elementary particles, that they are, moreover, dissymmetrical, and that, in short, the dissymmetry of the one group is the same as that of the other, but in an inverse sense. If, for example, the arrangement of the atoms of the right-handed tartaric acid present the exterior appearance of an irregular pyramid, the arrangement of the atoms of the left-handed tartaric acid ought, of necessity, to present the form of a pyramid irregular in the inverse sense. II. Nominated assistant professor of chemistry at Strasburg, Pasteur followed up with enthusiasm these curious studies. To interrupt them for an instant it required nothing less than his engagement with Mademoiselle Marie Laurent, daughter of the Rector of the Academy. It is even asserted that on the very morning of his marriage it was necessary to go to his laboratory and remind him of the event that was to take place on that day. But if Pasteur was thus guilty of an absent-mindedness worthy of La Fontaine, he proved as a husband so different from La Fontaine that Madame Pasteur, when reminded of this lapse of memory, receives the reminder with an indulgent smile. But to return to the laboratory: Under the same conditions of weight, temperature, and quantity of solvent, Pasteur placed successively, in presence of the two acids, all the substances capable of combining with them. In this way he obtained right-handed and left-handed tartrates of potash, of soda, of ammonia, of lime, and of all the oxides properly so called. He applied himself to the compounds--and they are numerous--which deposit themselves in liquids under well-determined crystalline forms. Without entering into the details of these long and patient studies, it may be stated generally that Pasteur proved that whatever could be done with one of the tartaric acids could be repeated rigorously, under similar conditions, with the other, the resultant products manifesting constantly the same properties, with the single difference already exhibited by the two acids--that in the one case the deviation of the plane of polarisation was to the right, while in the other it was to the left. With regard to all their other properties, both chemical and physical, the identity was absolute. Solubility, simple refraction by solutions, double refraction by crystals, the action of heat in producing decomposition, &c., the similitude extended to the most perfect identity.