REAGENTS. 17 



B. Miero-ehemical Reagents. 



Besides the fluids which are used for hardening 

 and staining the tissues, a considerable number are 

 employed, which, on account of the characteristic 

 effects produced by their action on cell-walls and cell- 

 contents, may be regarded as chemical tests for the 

 various substances which may be present. The follow- 

 ing are the principal reagents which are used in this 

 way : the mode of preparing them is also given, and 

 some indication of their uses; but this latter subject is 

 more fully treated in the next chapter. 



I. Acids. 



Sulphuric acid. This is used either concentrated 

 or dilute (1 to 3 of water). It causes, in either case, 

 the swelling-up of cellulose cell-walls, starch-grains, 

 &c. ; when cellulose cell-walls which have been pre- 

 viously saturated with solution of iodine are treated with 

 sulphuric acid, they turn blue. 



Concentrated sulphuric acid dissolves cellulose and 

 starch, but cuticularised cell-walls and the middle 

 lamella of lignified cells resist its action. It is used 

 with cane-sugar, as a test for proteids, and with aniline 

 sulphate as a test for lignin. 



Nitric acid. It colours cuticularised cell- walls and 

 proteids yellow ; it also causes swelling-up of cellulose 

 and of lignified cell-walls. It is useful for dissolving 

 the crystals of calcium oxalate which are frequently 

 present in the cells. It is used with ammonia as a test 

 for proteids (xanthoproteic reaction), and with potassium 

 chlorate as a test for suberin, and as a macerating fluid. 



C 



