As soon as the temperature has become reduced to that at which the particular modification of cholesteryl acetate is produced which forms liquid crystals, spots of light make their appearance at various points in the field, and each expands into a beautiful circular and more or less coloured disc marked by a rectangular sectorial black cross, which latter is well shown in the illustration (Fig. 121). These beautiful apparitions continue to occur, and each to expand to a certain size, which is rarely exceeded, until the whole field becomes filled with the wheels or crossed discs, the general effect very much in some respects resembling that afforded by a slide of the well-known polarising substance salicine. These discs, however, are liquid, being spherical drops, of the structure already described and illustrated in Fig. 103, and that this is so is at once made apparent on touching the cover-glass with a pen-knife or other hard pointed substance, which immediately causes them to become distorted. They recover instantly their shape again when the pressure is removed. When the cooling, moreover, has proceeded still further, there is a sudden change, and acicular solid crystals shoot over the screen, tinted with all the colours of the spectrum, until the field is full of them, the ordinary solid modification of the substance having then been produced. The experiment may be repeated with the same specimen of the substance, mounted on the same slide, covered with the usual thin cover-glass, time after time for months, at reasonable intervals. In concluding this chapter it may be mentioned that absolute proof of the double refraction of the liquid crystals of several different substances, derivatives of cinnamic acid, has been afforded during the year 1910. For direct measurements have been carried out by two independent investigators, Dorn and Stumpf, of the two refractive indices corresponding to the ordinary and extraordinary rays in each case, the crystals being uniaxial. CHAPTER XVII THE CHEMICAL SIGNIFICANCE OF CRYSTALLOGRAPHY. THE THEORY OF POPE AND BARLOW—CONCLUSION. Nothing in connection with the subject of crystallography is more surprising than the neglect and apathy with which it has been for long treated by the chemical world. That crystalline structure is intimately related to chemical constitution will have been made abundantly plain during the course of this book. Yet in spite of the great work of Mitscherlich, essentially a chemist, and of a large amount of striking work which has been steadily accumulating during the last thirty years, with results of vital importance to chemistry, it is only at the eleventh hour that chemists are really awakening to the vast significance which crystal structure has for them.