14 INTKODUCTION. 



same treatment, especiallj- if the slide is slightly warmed, colors 

 the so-called intercellular substance yellow. The acid is also 

 used as a test for suberin (see 158). 



66. Osmic acid (perosmic acid) is very volatile, and there- 

 fore is best preserved in sealed glass tubes until wanted for use, 

 when the tube can be broken under water. Even from the aque- 

 ous solution the irritating acid escapes in small amount, render- 

 ing it a disagreeable reagent to work with. The solutions are 

 usually of one per cent strength. 



Oils are colored biown by the reduction of the acid to me- 

 tallic osmium on the surface of the drops. Living protoplasm 

 is killed at once by even dilute solutions of this acid, and there 

 is usuallj- more or less discoloration of the different parts. 

 Hence it is a useful agent for arresting the processes of cell- 

 division and growth at any desired stage. Advantage is some- 

 times gained, according to Poulsen,^ by the combination with it 

 of chromic acid. 



67. Phenol (see carbolic acid, 52). 



68. Phloraglucin^ used bj' Wiesner as a test for lignin.' 

 The specimen is first acted on by hydrochloric acid, and then 

 moistened by a solution of phloroglucin in water or alcohol. If 

 the cell-walls are lignified, the}' will at once assume a red color. 

 HiJhnel'' suggests the employment of a strong decoction of cherry 

 wood instead of the phloroglucin. Used in the same way, it im- 

 parts a violet color to lignified cells. This test is hardly so 

 satisfactorj- as the other. 



69. Potassic bichromate in aqueous solution is used to harden 

 tissues, and is about as good as chromic acid. It has been also 

 emploj-ed by Sanio* for the detection of tannin. 



70. Potassic chlorate, used with nitric acid, is the most con- 

 venient maceiating agent. If a few small crystals of this salt 

 are added to a little concentrated nitric acid in a test-tube con- 

 taining a fragment of wood, and liie liquid is carefully warmed, 

 violent action begins somewhat below the point of boiling, and 

 the wood is speedily disintegrated. By selecting acid of the 

 right strength, and by careful regulation of the heat applied, the 

 action of the liquid can be kept well under control, so that 

 almost any degree of action can be obtained. It is not safe to 

 use this reagent in the room where delicate apparatus is kept, 



^ Miliroohemie, p. 19. 



2 Sitziingsber. Akad. Wien, 1878, p. 60. 



8 H). 1877, p. 685. 



1 Bot. Zeitnng, 1863, p. 17. 



