Growing Crystals by Instantaneous Photomicrography. 495 



attained. Since these larger images are not much more clear 

 than the smaller ones, while they occupy much more space, 

 the plates herewith given are all from the original negatives. 



A number of good impressions of crystallizing potassic 

 iodide were taken under these circumstances, but many other 

 rolls were sacrificed. The chief difficulty, as before, was to 

 secure the right moment; and this difficulty was of course 

 much augmented by the limited expanse of the field. Prints 

 from a few of the successful negatives are given below. In 

 order to give a clear impression in printer's ink, these were 

 much intensified by successive photographic printing and 

 intensification ; but of course no attempt was made to remove 

 the imperfections of the successive plates, for which allowance 

 may easily be made. (Plate IX. figs. 10-15.) 



The study of these photographs reveals several interesting- 

 points. In the first place, it is noticeable that no image is 

 wholly without evidence of crystalline structure. The most 

 doubtful cases are those in figs. 9 and 11 ; but the elongated 

 shape of these doubtful images seems to indicate a solid. A 

 globule of a new liquid phase, j-^jj-^ millimetre in diameter, 

 would have left an unmistakably circular image on the highly 

 magnified plate, for its index of refraction could not have 

 been identical with that of the aqueous solution. The fact 

 that we could not find such a globule of course does not prove 

 that a globule cannot exist, either for an infinitesimally brief 

 period of time, or of an infinitesimal magnitude beyond the 

 reach of microscopic observation. Nevertheless, so many 

 scores of photographs were taken as to diminish considerably 

 the probability that such globules can ever be seen with sub- 

 stances possessing a high melting-point. 



A striking fact to be noticed in nearly all the most highly 

 magnified records is the ill-defined appearance of the smallest 

 crystals. This appears to be due, not to a lack of structure, 

 but rather to the rapid growth in diameter which is manifest 

 in the young crystal. The initial rapidity is so great that the 

 fifth of a second appears to include several different stages of 

 growth, and hence a blurred impression results. It is easy 

 to obtain some idea of the rapidity of this initial growth by 

 comparing the sizes of the first two or three appearances of 

 each crystal. 



With this object a few of the series were measured by 

 means of an accurate micrometer ; but the conditions of the 

 experiments are too uncertain to give the very precise 

 measurements much value. Perfect constancy of tempera- 

 ture and evaporation, as well as in the rate of the revolving 

 shutter, involving grave complications in the apparatus, 



