ABSORPTION 151 



EXPERIMENT. Repeat the preceding experiment, but using potas- 

 sium nitrate in strengths .5, .25, .20, .19, .18, .17, .16, .15 normal; 

 incidentally compare rapidity and completeness of plasmolysis caused 

 by corresponding strengths of sugar and potassium nitrate, but espe- 

 cially observe which strength just produces plasmolysis, and therefore 

 exactly equals the exosmotic power of the crucial strength of cane- 

 sugar. 



EXPERIMENT. Repeat the two preceding experiments with a 

 colored cell-sap, preferably the now classical red epidermis of Rhoso 

 (or Tradescantia) discolor, or else red Beet, or Tradescantia zebrina, 

 Determine the solutions of cane-sugar and potassium nitrate which 

 are isotonic (or isosmotic). 



DEMONSTRATION METHODS. A method of projecting a plasmolyzing 

 cell upon a screen so as to be visible to an audience is described by PFEFFER 

 in his paper earlier cited (page 23, note). 



NORMAL SOLUTIONS. In these studies percentage solutions (i.e., those 

 containing the percentage of the substance in grams dissolved in enough 

 water to make up 100 grams) could of course be used, but, in view toth of 

 the nature of osmotic pressure and also of present usage in physics and chemis- 

 try, it is much better to employ equimolecular, commonly known as normal, 

 solutions. There is some diversity of usage with regard to them, but the 

 most logical, and that towards which usage seems to te tending, is this: a 

 normal solution is one containing one gram-molecule (viz., the molecular 

 weight in grams) of the substance in enough water to make one liter of solu- 

 tion. In practice it is made by placing the proper weight of the substance 

 (thus: of cane-sugar 342 grams, of potassium nitrate 101 grams) in a suit- 

 able measuring-flask with enough distilled water to dissolve it, and then 

 adding enough distilled water, all at standard temperature, to make a total 

 volume of one liter. Obviously solutions thus made contain, volume for 

 volume, the same number of molecules of solute regardless of its chemical 

 composition. Some students, however, especially formerly, simply dis- 

 solved a gram-molecule of the substance in one liter of water, which made 

 the total volume different for different substances, and hence not strictly 

 equimolecular, volume for volume. On the other hand some chemists pre- 

 fer a somewhat different usage, and one of much less value for our present 

 purposes, as follows: a normal solution is one containing one gram equiva- 

 lent (viz., the weight in grams which will react with a gram-molecule of a 

 monovalent compound) of the substance in enough water to rrake one liter 

 of solution. Thus of H 2 SO 4 , not 98 grams would be used, but 49; and 

 therefore the gram-equivalent normal solution may contain either the same 

 amount or , J, or i the amount of a gram -molecule normal solution. 

 The two normal solutions may be distinguished respectively as molecular 

 normal and equivalent normal, or, better, following ARRHENIUS, the former 

 may be distinguished as N, while the latter are distinguished as N, N$, etc., 

 the two kinds being of course identical in monovalent substances, as well 

 as in all neutral organic compounds. The normal solutions when diluted 



