INTRODUCTION. 15 



since the gases evolved act upon metals. This is Schulze's 

 macerating process. 



71. Potassic nitrate, 1 used in the examination of proto- 

 plasm (see Part II.). 



72. Rosolic acid, or corallin, dissolved in water containing a 

 trace of sodic carbonate, forms a purple fluid which colors vege- 

 table mucus red. It is used also to demonstrate the structure of 

 cribrose-tissue. 2 



73. Schweizr.r*tf reagent (see cuprammonia). 



74. Sodic cfdoride (common salt), used in aqueous solution 

 in the examination of protoplasm (see 120). 



75. Sugar. Cane sugar dissolved in water to form a thick 

 syrup is allowed to act for some time on tissues containing pro- 

 toplasm : a drop of concentrated sulphuric acid is then placed 

 on the object, when the protoplasm will take on a faint rose-red 

 color. The reaction is uncertain. 



76. Sulphuric acid. Pure concentrated acid is used as an 

 adjuvant in many tests, e.g., with iodine solutions in the identi- 

 fication of cellulose, but it is also of great use by itself in break- 

 ing down cellulose. By it, a cellulose wall can be destroyed 

 without destruction of the protoplasm within (see 141). 



77. Staining agents. A few of the chemicals in the foregoing 

 list impart to certain tissues, and certain contents of cells, colors 

 which have a good degree of permanence when the specimens 

 are preserved in a suitable medium. But the colors produced 

 by most reagents are fugitive, and serve only a temporary pur- 

 pose. When, therefore, it is desirable to stain or tinge a given 

 part of a specimen permanently, recourse must be had to dyes 

 which do not readily fade. 



78. Some of these have been long in use in Vegetable His- 

 tology for the purpose of preparing attractive specimens for the 

 demonstration of tissues, but it is only within a recent period that 

 the}* have been successfully employed in the stud}" of cell-divi- 

 sion. In the examination of the changes which take place in 

 the interior of cells during division, they are indispensable : in 

 the examination of the tissues themselves, their use is fur from 

 satisfactory. As will be specially shown later, the chemical 

 differences between the cell-walls of certain tissues which it is 

 desirable to distinguish from each other under the microscope 

 are not very great, and they often behave alike as respects 



1 Treub: Naturk. Verh. d. koningl. Akad. vol. xix., 1878, p. 9. 



2 Behrens: Hilfsbuch, p. 313. 



