T. C. WHITE ON THE CRYSTALLIZATION OF HIPPURIC ACID. 45 



spread, the attraction of cohesion is locked up, but it soon strug- 

 gles to be free, and by the absorption of moisture from the atmos- 

 phere it regains its power, and dots of circularly forming crystals 

 begin their growth all over the film. This result can be obtained 

 more quickly by holding the slide a little distance above a lamp, 

 or it may be varied by breathing on it, or by alternating each of 

 these processes, or even by alternately heating and cooling the 

 slide at a few seconds interval ; by these means Hippuric Acid 

 may be made to assume an indefinite number of concentric forms, 

 which under the polariscope exhibit a gorgeous play of colours. • 



One striking variety occurs if the crystals are formed at a tem- 

 perature above 800° ; the crystals then become somewhat of the 

 natural circular form, but arranged spirally without going through 

 the usual preliminary film, and they do not display much colour 

 under polarisation. These are interesting in relation to the ten- 

 dency of some salts to crystalise spirally at high temperatures. 



The slides most interesting are those formed at a temperature of 

 about 100°; they will present the most beautiful and varied forms 

 of this Acid, and will furnish crystals that will be much admired 

 either under dark ground illumination or polarised light. Although 

 I have exhibited slides of Hippuric Acid upon one occasion at an 

 extra meeting, I thought it would be as well to bring the subject 

 forward at an ordinary meeting when we have a larger attendance, 

 and when I could receive suggestions from others who have given 

 more attention to the subject of crystallization generally, and when 

 I could answer any questions that might be put to me relative to 

 this Acid and the method of preparing this favourite slide for the 

 polariscope. This must be my apology for bringing forward so 

 elementary a paper on this occasion. 



Extra Meetings. — Members of the Club are reminded of the 

 Extra Meetings, which take place on the Second Friday evening in 

 each month, at Seven o'clock. They are devoted to the exhibition 

 of objects, and general conversation on microscopical subjects. 

 During the Excursion season the results of the previous excursion 

 generally contribute to the interest of these meetings. 



Journ. Q. M. C. No. 10. e 



