MICROCHEMISTR Y. 1 29 



methods heretofore used for the microscopical recognition 

 of the proteids in general, and then to describe a few related 

 substances which have been microscopically recognized in 

 the protoplasm. 



a. Reactions of the Proteids. 



a. Solutions of Iodine. 



224a. With iodine the proteids take a yellow or brown* 

 color, according to the strength of the solution, and this 

 reaction has been much used for their recognition, although 

 many other substances give the same reaction. But in 

 many cases the behavior of a doubtful body with iodine 

 may give a clew to its nature. It is best to use a solution 

 of iodine and potassium iodide containing more iodine than 

 is used for the recognition of starch, since iodine is taken up 

 much less freely by proteids than by starch. 



ft. Nitric Acid. 



225. Ordinary concentrated nitric acid gives a yellow 

 color with proteids by the formation of the so-called xantho- 

 proteic acid. This reaction may be hastened by gentle warm- 

 ing. The color becomes considerably deeper by the addition 

 of caustic potash or ammonia, since the xanthoproteic salts 

 of potassium and ammonium are more deeply colored than, 

 the free acid. But this reaction is not entirely trustworthy,, 

 since not only do tyrosin and various oxy-aromatic com- 

 pounds give the same reaction, but also certain oils, resins, 

 and alkaloids (cf. Nickel I, 17). 



y. Milton's Reagent. 



226. The so-called Millon's reagent is a mixture of mer- 

 curic and mercurous nitrates and nitrous acid. It is best pre- 

 pared, according to Plugge (I), by dissolving one part by 

 weight of mercury in two parts of nitric acid of specific 

 gravity 1.42, and then diluting it with twice its volume of 

 water. According to Nickel (I, 7), it may be prepared by 

 dissolving I ccm. of mercury in 9 ccm. of concentrated nitric 



