'We must,' said Pasteur to me on the day when, starting from the note of Mitscherlich, he passed all these things in review, 'we must invoke the action of solenoid or helix. Entangled at present in labours more than sufficient to absorb whatever of ardour and of force still remains to me, I have no longer time to occupy myself with these questions.' But what great things are to be done in following out this order of ideas, and what a route will be opened to young men possessed of that genius of invention which is evoked so often by persistent work! * * * * * This complete opposition between artificial mineral products and vegetable and animal ones was to Pasteur a truth so well established that he found frequent opportunity of affirming it under decisive circumstances. One day, a very skilful chemist, M. Dessaignes, who later on became one of the correspondents of the Academy of Sciences, announced that he had transformed fumaric and malic acids into aspartic acid. Pasteur, who some time previously had had occasion to study these same acids, had proved that the two first had no molecular dissymmetry--that is to say, they exercised no optic action. In the state of solution they did not turn the plane of polarised light. Aspartic acid, on the contrary, had presented to him molecular dissymmetry, like asparagine itself. If the observation of M. Dessaignes were true, then bodies which were inert in regard to polarised light, and consequently non-dissymmetric, could be transformed in the laboratory into active dissymmetric bodies. The line of demarcation so well established would be broken. Pasteur, whose experience regarding the note of Mitscherlich had shown him how even the most conscientious observers may fail to seize upon fugitive appearances, when unprompted to seek them by a preconceived idea, doubted at once the accuracy of the facts cited by M. Dessaignes. From Strasburg he started for Vendôme, where M. Dessaignes at that time resided. M. Dessaignes immediately gave Pasteur a small quantity of the aspartic acid which he had prepared by means of fumaric and malic acids. Returning to his laboratory, Pasteur immediately recognised that, despite the very close resemblance of the new acid of M. Dessaignes to that derived from asparagine, the former differed from the latter by the complete absence in its case of molecular dissymmetry. With regard to other facts of the same kind, announced not only in France, but in Italy, and in England--chiefly the pretended formation of grape tartaric acid from succinic acid, artificial and inert, by Perkin and Duppa--Pasteur testified with absolute certainty of judgment to the existence of phenomenal peculiarities proper to these substances, which he had never seen, and which had, on the other hand, been the object of careful study by observers of great talent.