J. M. Blake — Growing Crystals for Measurement. 567 



Art. XXXYIII. — Growing Crystals for Measurement; by 

 John M. Blake. 



When we have soluble salts to work upon we can, by 

 proper treatment, grow crystals that are nearly symmetrical 

 and complete in their parts, and that have the correct propor- 

 tional development of planes belonging to the species. We do 

 not as a rule, have this advantage with mineral species. We 

 have to take the crystals as we find them. It is comparatively 

 rare that complete crystals are found, and very often, the 

 individual crystals are crowded together, so that, besides losing 

 one end of each crystal, we have more or less distortion of 

 form, and miss the correct proportioning that free-growing, 

 suspended crystals should assume. 



It is with the object of drawing attention to the importance 

 of using free-grown crystals for the study of the laws that 

 govern all crystal development, that this is written. Such 

 crystals can easily be grown from solutions. 



There are questions in regard to crystal-growth and develop- 

 ment that seem important to test by experiment, and although 

 some work has been done in this direction, the field appears 

 worthy of further exploration. Some methods for growing 

 will be given. Individual efforts in this direction will doubt- 

 less suggest improvements in apparatus. 



The solutions of all salts can become supersaturated. The 

 maintenance of this condition in proper degree has an im- 

 portant influence on the quality of the crystals obtained. 

 It means a reserve of material which may require some time 

 to deposit, and there are certain limits of saturation within 

 which this reserve material will act selectively to the extent 

 that it will favor our growing crystal without forcing other 

 and undesirable crystals to start in growth. We have to wait 

 for diffusion to take place to bring the material to the place it 

 is wanted, and diffusion is slow through the bulk of the 

 solution. 



Control of temperature is of the first importance in gaining 

 and maintaining the proper degree of supersaturation. If we 

 cool a hot saturated solution, and the salt is being deposited 

 rapidly, a crystal suspended in it will have an upward current 

 flowing from it of solution that has become lighter from loss 

 of deposited material. On the contrary, if we suspend a 

 crystal in an under-saturated solution, a current of dense 

 solution will flow downward from the crystal as it slowly 

 dissolves. During a very rapid deposition of material there 

 will be a lively commotion from the ascending currents. It 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 233.— May, 1915. 

 37 



