124 SCIENCE PROGRESS. 



Again, to take another example, Pope has recently de- 

 scribed the behaviour of Chloral Hydrate which, when 

 melted on a microscope slide and allowed to cool, solidifies 

 as a thin crystalline film consisting of needles of a uniaxial 

 substance ; if this be allowed to stand, biaxial plates begin 

 to make their appearance among the needles and continue 

 to grow until the latter entirely disappear, the uniaxial being 

 entirely converted into the biaxial modification. 



But between such substances and the liquid drops of 

 Gattermann's preparations the difference is great. Some- 

 thing like a transition between the two is supplied by the 

 Benzoate of Cholesteryl discovered by Reinitzer and studied 

 by Lehmann in 1889, shortly before the other three yet 

 more remarkable compounds came under his notice. 



When the benzoate is warmed to i78"5° it melts to a 

 clear liquid, and if this be allowed to cool slowly it becomes 

 transformed into a dull white aggregate ; at 145° true 

 crystals make their appearance in this and continue to grow 

 rapidly at the expense of the white mass, but when warmed 

 they are again converted into the white substance. The 

 granules of which the latter consists can be conveniently 

 examined if the temperature be maintained at 178°, and 

 though not absolutely liquid are found to be so plastic 

 that they can be moulded and made to run into each 

 other. 



Lehmann's interpretation of these bodies was called in 

 question by Quincke (6), who suggested that their behaviour 

 was due to the presence of oil ; but Lehmann has repeated 

 the experiments with extra precautions and has further de- 

 scribed the still more remarkable features of soft crystals 

 of potassium oleate, which orientate themselves, come to- 

 gether in parallel positions and unite to form a single 

 crystal. 



Ordinary crystals are solids which can be deformed up 

 to a certain point without any permanent change being 

 produced in their form ; they may, for instance, be bent by 

 pressure and then return to their original form when the 

 pressure is removed ; unless the limit of elasticity has been 

 passed, when they are permanently deformed or broken. 



