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On Microscopic Crystals formed at High Temperatures. 



By T. HooKHAM. 

 (Bead May 22nd, 1868.; 



(The original source of this paper is to be found in an article by- 

 Mr. Thomas, of Oxford, printed in the " London Microscopical 

 Journal" for July, 1866, and copied into the last edition of Dr. 

 Beale's work on the Microscope.) 



The following is a theory of " high temperature " crystals. 



It is found that hydrous crystals, as prepared for the microscope, 

 assume various forms according to the temperature at which they 

 are crystallised. A solution is placed upon a glass slide, and 

 rapidly evaporated over the flame of a spirit lamp. If this opera- 

 tion is properly conducted (and this is the first difficulty the ama- 

 teur will encounter) a transparent film, much like a gum in appear- 

 ance, is formed upon the glass. This film would appear to consist 

 of all the constituent atoms of a crystal, with the exception of the 

 atoms of water, without which, as is well known, the hydrous 

 crystals cannot be formed. So long as the slide retains its high 

 temperature the moisture in the atmosphere cannot condense upon 

 it, and consequently crystallisation cannot take place. But it is 

 obvious that by varying the temperature of the slide we can so 

 regulate its relation to the temperature of the atmosphere that we 

 can supply water to it by condensation in such quantity as we deem 

 advisable. This, therefore, is the method of treatment. We take 

 the uncrystallised and (so long as it remains anhydrous) imcrys- 

 tallisable film, and subject it to a given temperature. If this tem- 

 perature is sufficiently low it will absorb a certain amount of mois- 

 ture from the atmosphere, and crystallisation will commence in a 

 shorter or longer time, according, probably, to the proportion of 

 water necessary to the formation of the particular crystal. If the 

 temperature is low, say 60°, what is called a foliated crystal will be 

 formed upon the slide, and this in a short space of time, perhaps 

 from five to ten minutes. Having succeeded with this temperature, 

 we may now try a higher, say 70°. A longer time will be necessary 

 to crystallisation, and a different form of crystal will be formed. 

 (Engravings of such crystals, taken from the accurate drawings of 

 Mr. Thomas, of Oxford, will be found in the " Microscopical 

 Journal," as above referred to, and in Dr. Beale's last edition.) 



