50 BOTANICAL MICROTECHNIQUE 



or into heaps of red granules on the addition of platinum 

 vhloride. 



4. Sodium and potassium sulphates may often be recog- 

 nized in the living tissues by means of nickel sulphate. 

 With this they form well crystallized double salts of the 

 composition NiSO 4 + Na 2 SO 4 + 6H,O (or the correspond- 

 ing K salt) ; these occur mostly in the form of the mono- 

 clinic prism combined with the basal plane, but are pretty 

 easily soluble in water. 



6. Nitric Acid, HNO 3) Nitrous Acid, HNO 2 , and their Salts. 



73. DipJienylamine was first recommended by Molisch (I) 

 for the recognition of the nitrates, and he used for fresh 

 sections a solution of from T ^ to y 1 ^ of a gram of it in 10 

 ccm. of pure concentrated sulphuric acid, or for dried sec- 

 tions a concentrated solution of it in concentrated sulphuric 

 -acid. In the presence of nitrates there occurs immediately 

 after the addition of this reagent a deep-blue coloring 

 which, after a time, disappears or passes into brownish 

 yellow. 



This reaction occurs in the same way in the presence of 

 nitrites, and it can therefore be used for the recognition of 

 nitrates only when the absence of nitrous salts is proved. 

 But in fact all investigations on the subject heretofore 

 liave led to the conclusion that nitrous salts do not occur 

 within the living plant ; and therefore this objection to the 

 applicability of diphenylamine as a reagent for nitrates falls, 

 so far as the microchemical study of the plant is concerned. 



'It should be remarked that other compounds than nitrates 

 and nitrites give the same reaction, as, for example, man- 

 ganese peroxide, potassium chromate and chlorate, hydro- 

 gen peroxide, ferric oxide and its salts (cf. Frank I and II, 

 and Kreusler I). But these substances appear to be as 

 rare in the plant as nitrites ; at all events, plants freed from 

 nitrates never give a blue color with diphenylamine, accord- 

 ing to the confirmatory researches of Frank and Schim- 

 per(II, 217). 



