Organic Substances 65 



until the background is a pale yellow-brown. Overdifferentia- 

 tion may weaken the reaction proper. 



Oxidation by chromic acid followed by SchiflF's reagent is 

 Bauer's method;^^ oxidation by periodate followed by Schiff's 

 reagent is the Hotchkiss-McManus method."^' ^^ 



Etcheverry and Mancini^^ have described a method for 

 carbohydrates based on the fact that tungstic acid, on expo- 

 sure to ultraviolet light, is reduced by carbohydrates to a 

 blue oxide of unknown composition. The mechanism of the 

 reaction is not clear. The authors believe that it involves the 

 oxidation of glycols to aldehydes. 



The method is mentioned only as a curiosity; it cannot 

 compete with other techniques which give a much better 

 color contrast. 



Specificity of aldehyde reactions after oxidative procedures. 

 —As mentioned before, Schiff's reagent applied to paraffin or 

 celloidin sections following the oxidative procedures de- 

 scribed is specific for vicinal OH or OH and NH2 groups, 

 which occur only in carbohydrates and in a few amino acids. 

 The same appHes to the silver reagent after chromic acid, 

 since substances which would reduce it without previous 

 treatment (uric acid, phenols, melanin and its precursors) 

 are destroyed by oxidation. 



Two exceptions, however, must be made. First, under cer- 

 tain conditions, both normal and pathological, unsaturated 

 fatty acids of the tissues may undergo changes resulting in 

 the formation of peculiar substances the exact nature of 

 which is not clearly understood. They exhibit the character 

 of aldehydes and/or peroxides; most, but not all, of them are 

 removed by the procedures of dehydration and embedding. 

 They may be fairly well preserved in paraffin sections after 

 prolonged fixation in bichromate-containing mixtures and 

 give a positive SchiflF reaction, to some extent even without 



41. Etcheverry, M. A., and Mancini, R. E.: Rev. Soc. argent, de biol., 

 24:156, 1948. 



