10 PASTEUR: THE HISTORY OF A MIND 



inclined to the right, the crystal being oriented in a 

 manner agreed upon; left-handed plagihedrons, those 

 in which it inclined to the left. There he rested the 

 matter. His pupil, Delafosse, had likewise seen in 

 these crystal-facets only the confirmation of his ideas 

 respecting the tetrahedral character of the integral mole- 

 cule. If we imagine a series of tetrahedrons, threaded end 

 to end along a rigid rod, this thread will terminate at 

 one end in a point, at the other end in a plane; the one ex- 

 tremity corresponded, for Delafosse, to the corner which 

 was not truncated and remained pointed, the plane sur- 

 face to the other extremity bearing the hemihedral facet. 

 For some years the discovery of Biot and that of 

 Haiiy existed side by side in science without influencing 

 each other. It was John Herschel who applied to 

 this inert machinery the drop of oil destined to make it 

 go. He bethought himself of combining the purely 

 crystallographical observation of Haiiy on the right- 

 and left-handed plagihedrons with the purely physical 

 observation of Biot on the right- and left-handed 

 quartz. Since one defines arbitrarily the crystallo- 

 graphical position of the crystal of quartz which he 

 examines, it is possible to place the crystal in such a 

 way that the right-handed quartz shall be also the right- 

 handed plagihedron, and the left-handed quartz, the 

 left-handed plagihedron. Thus there appeared to be 

 a connection between the crystalline form and the direc- 

 tion of the rotation. Observe that this arbitrary 

 definition which we have just made is not at all obliga- 

 tory and may be replaced by the opposite one. 

 What is essential is that the existence of the rotary 

 power was put by Herschel into relation with the 

 inequalities in construction of the crystal, and that 

 alongside of the different but nevertheless similar 

 structures which the existence of the right-handed 



