INTRODUCTION. 13 



without. It must be remembered that acetic acid dissolves 

 carbonates, but not oxalates (see 36). 



This acid has been used by Pringsheim l in the study of 

 chlorophyll grains ; fresh sections of tissues containing chloro- 

 phyll being exposed to the action of the acid for some hours. 

 From the grains, minute spheres of a brownish color become 

 nearly detached, and these afterwards appear as clusters of 

 acicular crystals (see Part II.). Hydrochloric acid is also of 

 use in the examination of some protein matters (see 124). 



62. Indol (Niggl's test 2 for lignin) is used in an aqueous so- 

 lution. The specimen, subjected to the action of the solution for 

 a few minutes, is transferred to sulphuric acid of specific gravity 

 1.2 (made by adding one part of concentrated acid to four parts 

 of water). Lignified structures become red. 



63. Mercuric c/iloride, or corrosive sublimate, dissolved in 

 fifty parts of absolute alcohol renders protein grains insoluble 

 in water. Pfeffer 8 recommends that the specimen should remain 

 in this reagent at least twelve hours. Dippel 4 uses a dilute 

 aqueous solution (1 in 500) to render visible the currents in the 

 most delicate threads of protoplasm (and for the demonstration 

 of the nucleus without affecting the other contents of the cell). 



64. MHlorfs reagent, common!}' called acid nitrate of mercury, 

 is best prepared, according to its discoverer, by pouring upon 

 pure mercury its own weight of concentrated nitric acid. For 

 a short time the action is violent ; when it subsides a little, 

 gently warm the liquid until the metal is completely dissolved. 

 The solution is Immediately diluted by twice its volume of pure 

 water. After a few hours the liquid is to be decanted from the 

 crystalline mass which has formed, 'and it is then ready for use. 6 



This reagent is more efficient when freshly made. 



Albuminoid substances are colored jred by this reagent even 

 in the cold, but much more readily upoii the application of heat. 

 According to Millon, the reaction is due to the presence in the 

 liquid of both mercuric nitrate and nitrite. 



This reagent has been employed for the demonstration of the 

 stratification and spiral striation of certain cell-walls. 



65. Nitric acid gives to protein matters a yellow color, 

 which is intensified upon the subsequent use of ammonia. The 



1 Pringsheim's Jahrbiicher, Bd. xii. p. 294, et seq. 



2 Flora, 1881, p. 545, et seq. 



8 Pringslieiiu's Jahrbiicher, viii. p. 441. 

 * Dippel: Das Mikroskop, i. p. 281. 

 5 Quoted from Behrens: Hilts!., p. 247. 



