13 BOTANICAL MICROTECHNIQUE. 



acid of specific gravity 1.52 and diluting the solution with 

 an equal volume of water. The reagent so obtained becomes 

 decomposed in time, and may then be restored, according to 

 Krasser (I, 140), by the addition of a few drops of a solution 

 of potassium nitrite. 



Millon's reagent gives with proteids a brick-red, or more 

 rose-red, color, which appears usually after some time in the 

 cold, but much more quickly on gentle warming, without 

 any solution of the proteids. The protein crystalloids in 

 the endosperm of Ricinus, for example, retain their form 

 unchanged even on heating nearly to the boiling point. 

 This reaction is also very delicate, but it has the disadvan- 

 tage that it takes place also with a large number of other 

 compounds, in the same way ; according to Plugge (I), with 

 all those that contain an OH-group on the benzol nucleus. 



227. Reactions similar to that of Millon's reagent are 

 given by Hofmanris reagent, which is a solution of mercuric 

 nitrate with traces of nitrous acid, and by the so-called 

 Plugge 's reagent, which consists of meicurous nitrate with 

 traces of nitrous acid (cf. Nickel I, 12 and 13). 



8. Raspail's Reagent. 



227a. The so-called Raspail's reagent consists of a con- 

 centrated aqueous solution of cane-sugar and concentrated 

 sulphuric acid, which are added to the objects to be tested 

 at the same time. Proteids are colored by this rosy-red or 

 somewhat violet. But the reaction does not succeed with 

 all proteids and does occur with various other substances. 

 Many glucosides and alkaloids are colored red by sulphuric 

 acid alone (cf. Nickel I, 37 fL). 







e. Copper Sulphate and Caustic Potash. 



228. Most proteids give a violet reaction on treatment 

 with copper sulphate and caustic potash ; but the reaction 

 is neither always very sharp nor, when it succeeds, positive 

 proof of the presence of proteids. The reaction may be 

 conducted in the same way as for cane-sugar (cf. 121). 



