178 ON VAEIABILITY AND ADAPTATION 



— by actual rupture of the crystals, the separate parts gliding the 

 one on the other. 



Undoubtedly this does happen with certain crystalline 

 substances, but Lehmann, in 1889, first called attention to 

 another order of phenomena. If solid cholesteryl benzoate, 

 which is in the form of colourless crystalline plates, be heated 

 to the temperature of 145 - 5° C, it melts into a turbid fluid 

 having the consistence of olive oil. Heated still further, as 

 Eeiuitzer first showed, at 178 - 5° C. it suddenly becomes a 

 perfectly transparent fluid. Studying the intermediate stage, 

 Lehmann found that under the polarization microscope the 

 turbid fluid, despite its fluidity, exhibited double refraction 

 with the crossed Nicol's prisms — a property hitherto regarded 

 as associated with the solid crystalline state only ; heated to 

 178 - 5° C, the field became dark and isotropous, like ordinary 

 fluids. If the opposite process were now undertaken and the 

 heated fluid subjected to cooling, the whole field became con- 

 verted into a mass of doubly refractive spherocrystals, showing 

 here and there little dark crosses ; cooled further, these gave 

 place to plates of the solid modification, which plates grew in 

 size until they occupied the whole field. 



In fairly rapid succession other substances were determined 

 having the same peculiarities : — other compounds of cholesterin 

 with the fatty acid series, such as cholesteryl acetate, cholesteryl 

 propionate, and cholesteryl oleate, compounds of oleic acid, 

 sodium, potassium and ammonium oleate, as also methyl-, 

 dimethyl-, and trimethylamin oleates, and various paraderiva- 

 tives of anisol and phenetol. In Table B I have transcribed 

 the list given by Sehenck last year, adding thereto certain com- 

 pounds found by us to possess the same properties, acknowledg- 

 ing that it is not complete, and that already numerous additions 

 have been made during the last few months. 



All these bodies become fluid on being heated, but the fluid 

 examined between crossed Nicol's prisms has still the main 

 optical features of crystalline substances ; it is anisotropous. 

 Some of these crystalline liquids are thick like olive oil, some 

 (like ^-azoxyphenetol) are more fluid than water. Poured into 

 a vessel the surface becomes level ; in tubes it assumes the 

 characteristic concave meniscus of fluid bodies, and this although 

 the constitution is crystalline. Heat to a further degree and 



