INORGANIC SALTS. 33 



Pf eff er r has used it in a two per cent, alcoholic 

 tincture in the study of protein grains. It unites 

 with these albuminoids, forming a compound insol- 

 uble in water ; but to secure satisfactory results 

 the preparation must lie at least twelve hours in 

 the fluid. 



CHLORIDE OF IRON. 



An aqueous solution may be employed as reagent 

 for tannin 2 when this is not present in too small 

 quantity. The cells to be examined, when placed 

 immediately in the reagent without the previous 

 contact of water, which easily removes the tannin, 

 assume a dark green or bluish-black color, accord- 

 ing to the nature of the tannin compound. The 

 green cells color yellow if potash is added. 



The solution should not be too concentrated, as 

 the tannate of iron which is formed is soluble in 

 an excess of this compound, and its prompt solu- 

 tion renders the test less evident. For this reason 

 acetate and sulphate of iron 3 have largely replaced 

 the chloride. Being more certain in their action, 

 they are to be preferred. 



1 Pfeffer : Jahrb. fur wiss. Bot., 1872, VIIL, p. 491. Weiss : Allg. Dot, 

 p. 140, note. Sachs: Lehrb., 1874, p. 55. Duchartre: Elements de Bot., 

 1877, p. 102. 



2 Karsten : Gesammelte Beitr. zur Anat. u. Phys. d. Pflanzen, L, 1865, 

 p. 253. Dippel: Das Mikroskop, I., p. 375. Wiesner : Technische Mikro- 

 skopie, p. 83. Weiss: Allg. Botanik., L, p. 181 ; Die Pflanzenhaare, Kar- 

 stens Bot. Unters., I. Nageli: Das Mikroskop, 1877, P- 475- 



3 Cf. Link : Grundlehrend. Anat., 1807, p. 80, where, to my knowledge, 

 it is mentioned for the first time as a micro-chemical reagent. 



