HOOKHAM ON MICR08C0PIC CRYSTALS. 81 



We may again try higher temperatures, till the limit is reached 

 beyond which crystallisation cannot take place at all ; and each time 

 we shall find a variously modified fonn of crystal to have been 

 produced ; each higher form possessing increasing interest for the 

 student. 



So far we have for the most part followed Mr. Thomas. He 

 found that about 110° was the greatest temperature at which he 

 could obtain a crystal, and even then its formation occupied many 

 hours. This practical difficulty cannot have been other than a 

 source of disappointment to him, for up to this point some of the 

 crystals steadily advanced towards a mathematical correctness of 

 form, and thus gave promises which the highest temperature he 

 could supply them with did not allow them to fulfil. The crystals 

 of sulphate of copper in particular exhibited such an advance to- 

 wards a mathematical form. Even the foliated crystals formed at 

 60° exhibit to the educated eye a twist in their lines, or a tendency 

 to the spiral form. At 70° he obtained a circular or " centrical " 

 crystal flowing out in waves from a fixed centre, and this still more 

 strongly showed the spiral twist. At higher temperatures the 

 waves became smaller, and the spiral more decided ; and having 

 with great difficulty obtained a crystal at 110° he found that the 

 ultimate tendency was, beyond a doubt, for the spiral to become 

 mathematically perfect, and the waves to vanish in pure lines. 

 Such a crystal he has accordingly figured, as he was fully justified 

 in doing, though he never obtained it absolutely perfect ; and it 

 may be added, the result has shown the sagacity of his prophecy. 



It occurred to the present writer that, after all, temperature was 

 only a means to an end, namely, the regulation of the supply of 

 moisture ; and though the most obvious, might possibly not be 

 found the only means. It appeared likely that in Mr. Thomas' 

 experiments with higher temperatures, it was not the small degree 

 of moisture, but the great degree of heat operating directly, which 

 prevented crystallisation. Since the direct influence of heat on the 

 formation of crystals is a fact well established, the problem, 

 therefore, appeared to resolve itself into this ; — how, either to 

 counteract the influence of the heat, or to gain the same power of 

 regulating the supply of atoms of water by other and more in- 

 direct methods, combined with a lower temperature. For a long 

 time this problem appeared insoluble. Months of experimenting 

 gave a uniform result of failure in the attempt to improve the 



