\VITII THE iMICROSCOrE. 21 5 



character. From the fact that throughout the series the crystals 

 radiate from centres in a more or less spiral manner, Mr. Thomas 

 has designated the process as " spiral crystallisation." 



The solution of sulphate of copper is evaporated by a moderate 

 heat, until an uncrystallised film is obtained. This film being sub- 

 jected (after the manner indicated in Mr. Thomas' paper, contained 

 in the Microscopical Journal for July, 1866, p. 177), to a temperature 

 of about 60 Fahrenheit, a number of foliated crystals, all radiating 

 from centres, appear. There is in this stage a slight curve or twist 

 in the radiation, and this constitutes the first stage of the spiral, as 

 represented in pi. XLVIII, fig. 306. At a temperature of 65, a 

 further advance is seen in the direction of the spiral, fig. 307. At 

 70 (fig. 308) the spiral appearance is yet more distinct. While at 

 temperatures of 80, 90 and 100, the lines are smaller and more 

 numerous, and the spiral more perfect and symmetrical, fig. 309. 

 Fig. 310 shows a perfectly formed crystal which had been allowed 

 to crystallise upon a slide, carefully protected from dust. Mr. Thomas 

 believes that these crystals are in reality cones standing out in relief 

 upon the glass slide. The changes in form which occur in crystals of 

 the double salt, sulphate of magnesia, sulphate of zinc, upon the appli- 

 cation of a gentle heat subsequent to crystallisation has been further 

 prosecuted by Mr. Thomas, of Oxford, whose figures are given in 

 pi. XLIX. See also Microscopical Journal for April, 1866, p. 137. 



3)15. Examination of Crystals under the Microscope. Some 

 crystals which have been entirely separated from 'the fluid in which 

 they were originally deposited, may be examined in the dry way, in 

 water, or other fluid in which they are known to be insoluble, or in 

 Canada balsam ; but as a general rule, it is necessary to examine the 

 crystals as they lie in some of the fluid in which they have been 

 formed. When they have been obtained by allowing a concentrated 

 solution to cool, some of the inspissated fluid must be removed with 

 the crystals, placed upon a glass slide or in a thin glass cell, covered 

 with a piece of thin glass, and examined in the usual way first using 

 a low power (an inch), and afterwards a higher power (a quarter), 

 because, although some of the crystals are of a large size, others 

 amongst them, the form of which is very perfect, are often exceed- 

 ingly minute. The crystals and mother-liquor should not be exposed 

 to the air previous to examination, for in many instances water is 

 absorbed, and partial solution takes place. 



316. Preservation of Crystals as Permanent Objects. The pre- 

 servation of the more soluble crystals is attended with the greatest 

 difficulty, except when dried, in which state their characters under 

 the microscope are not well defined. Crystals which very readily 



