At the moment when Pasteur, entering upon the labours which form the principal subject of this book, abandoned the study of molecular physics and chemistry which had previously occupied him, all his thoughts were directed to the search of means suited to render evident the influence of these causes and these phenomena. At Strasburg he had procured powerful magnets with the view of comparing the actions of their poles, and, if possible, of introducing by their aid, among the forms of crystals, a manifestation of dissymmetry. At Lille, where he was nominated Dean of the Faculty of Sciences in 1854, he had contrived a piece of clockwork intended to keep a plant in continual rotary motion, first in one direction and then in the other. 'All this was gross,' he said to me one day; 'but, further than this, I had proposed, with the view of influencing the vegetation of certain plants, to invert, by means of a heliostat and a reflecting mirror, the motion of the solar rays which should strike them from the birth of their earliest shoots, and in this direction there was more to be hoped for.' He never spoke of these attempts, because he had not had the time to follow them to the issues of which he dreamed; but to this day he remains persuaded that the barrier which exists between the mineral and organic kingdoms--and which is revealed to our eyes by the impossibility of producing, in the reactions of the laboratory, dissymmetric organic substances--can never be crossed until we have succeeded in introducing among these reactions influences of the dissymmetric order. According to Pasteur, success in this direction would give access to a new world of substances, and probably also of organic transformations. As we have succeeded in finding the inverse of right-handed tartaric acid, we may hope to obtain some day all the immediate principles inverse to those now known to us. Who could say what vegetable and animal species would become if it were possible to replace, in the living cells, cellulose, albumen, and their congeners, by their isomers with an inverse action? Certainly the thing is not easy, and Pasteur would be the last person to deceive himself as to the difficulty of the problem. His latest thought on the matter is this:--When the attempt is made to introduce into living species primordial substances, inverse to those now existing, the great difficulty will be to master the _tendency_ (_devenir_[7]) proper to the species, a tendency which is potential in the germ of each of them. In this germ, it is to be feared, the dissymmetry of the dissymmetric primordial substances which it embraces will always manifest itself. Ah! if spontaneous generation were possible; if we could form from mineral matter a living cell, how much more accessible would the problem become! However this may be, we must seek, by all possible means, to produce molecular dissymmetry by the application of forces which have a dissymmetric action.