HOOKHAM ON MICROSCOPIC CRYSTALS. 83 



be produced in quantities ; tliey occur only on a percentage of 

 slides. But the writer does not despair of eventually discovering 

 the law of their formation, since in some series of experiments they 

 occur much more frequently than in others. In the meantime, he 

 begs to present specimens of them to the Quekett Club for inspection, 

 if it will honour him by their acceptance. 



It only remains to add that by singular good fortune the process 

 adopted in the first place for the purposes above spoken of, also 

 serves another scarcely less important, and most efficiently. It not 

 only in all cases greatly facilitates the production of the " anhydrous 

 film," but it makes it possible in many cases where before it was 

 impossible. Thus an unlimited field is open in which to apj^ly this 

 method of crystallisation. The writer has already done this to 

 some extent Avith the most promising results. He is in treaty with 

 a scientific chemist to supply him with some of the hydrous crystals 

 recently discovered, and if the results answer his expectations he 

 will from time to time offer to the general and scientific public such 

 microscopic crystals as may seem in any especial manner to merit 

 their attention. 



The President spoke of the subject as being one of great interest, as leading 

 towards the discovery of the ultimate forms of crystals. 



Mr. Hislop said that he was not aware that the subject was to be brought 

 before the club that evening till the commencement of the meeting, but he had 

 himself brought a number of slides to shew some remarkable forms which ho 

 had obtained in the course of his experiments. He had for some time been 

 working upon crystallisation under various conditions, and had found that the 

 form of crystallisation was varied, not only by the temperature but by the rate of 

 cooling, or heating the slide. In preparing the slides, he used a mounting plate 

 of considerable thickness — a piece of half-inch boiler plate, heated by a gas or 

 spirit lamp flame applied to one end. Measuring the temperature was attended 

 with practical difficulties, as it could only be attained approximately, the tem- 

 perature of the film not being exactly that of the glass slide, the supporting 

 plate, or the air around. As an instance, he mentioned that merely placing a 

 cover over the slide while the crystallisation was going on, materially altered the 

 aggregate form of the crystals . If the salts or other substances in solution, 

 could be reduced to a viscid condition by a high temperature, and the slide 

 then dipped in some hydro-carbon — such as pure spirits of turpentine — the 

 crystals would be found more perfect in outline and details, and the effects pro- 

 duced when examined by polarised light were among the most gorgeous that 

 could possibly be conceived. He was still engaged on the subject, which was a 

 very wide one, and hoped to be able to present the result of his experiments to 

 the club. His own present conviction was, that we know very little about the 

 laws of crystallisation, and certainly not enough to warrant us in hasty generali- 

 sations relating to crystallography. 



