280 FATTY SUBSTANCES 



652. Polariscopic Examination. The following account is 

 based on Lison's (1933 a) conclusions. 



When examined in polarised light between crossed nicols fatty 

 substances may be : — 



(1) Isotropic {non-luminous). This result may be due to any 

 kind of fatty substance in liquid form : neutral fats and fatty 

 acids when liquid are never anisotropic ; cholesteryl esters, 

 phosphatides, and cerebrosides may be, bvit are not always, either 

 laecausc they are above their clearing points, i.e. the maximal 

 temperatures at which they can exist as liquid crystals, or because 

 the formation of liquid crystals is inhibited in some way. 



(2) Anisotropic (luminous). 



(a) Not showing a Black Cross of Polarisation. This result may 

 be due to any of the substances named under (1) when in solid 

 crystalline form. 



(6) Showing a Black Cross of Polarisation. This result may be 

 due to any of the substances named under (1) except neutral fats 

 and fatty acids and indicates the presence of liquid crystals. 



It is impossible polariscopically to distinguish between neutral 

 fats and fatty acids or between choleste'ryl esters, phosphatides 

 and cerebrosides. Polariscopic examination is thus not a method 

 for determining the composition of fatty substances, although it 

 is sometimes useful for studying their morphological distribution. 

 It is necessary for identifying stercl-digitonide in applications of 

 the Windaus test for free sterols. 



653. Methods Formerly Supposed to be of Histochemical Value. 

 By means of osmic acid it was formerly considered possible to 

 distinguish neutral fats from cholesteryl esters, phosphatides, and 

 cerebrosides. Osmic acid, however, does not react with all 

 fatty substances, and does react with many that are non-fatty ; 

 it is therefore not a reagent even for identifying fatty substances 

 in general, and has no histochemical value whatever (Lison, 1933 a). 

 After detailed critical examination (1926 a and h, 1928-29, 1932) 

 Kaufmann and Lehmann concluded that none of the following 

 methods was specific : Ciaccio's for " lipoids," Smith (Nile blue), 

 Fischler. and Smith-Dietrich. Lison has since found (1935 a, and, 

 with details, h) that commercial Nile blue is impure, containing, 

 in addition to true Nile blue, a substance that he terms Nile red. 

 The pink colour of fatty substances stained with commercial Nile 

 blue is due exclusively to this Nile red, which when pure stains 

 fat in the same way as Sudan ; no conclusion regarding the 

 composition of fatty substances is possible from the results with 

 either Nile red or pure Nile blue. These conclusions supersede 

 those Lison gives in his book (1936, p. 203). Lison (1933 a) pro- 

 posed the retention of the Smith-Dietrich method as being specific 

 for phosphatides and cerebrosides provided that cholesterol and 

 its esters could be excluded. He assumed that this condition 



