550 Btiller.—Contributions to our Knowledge of the 
burst more readily and in greater number when the pro- 
thallia are placed in water. 
In making the solutions of salt to be tested six concen¬ 
trations of potassium nitrate were used as standards, namely 
i j tV? tffj tftrtj too~o~o> an d tWoff gram-molecules 1 of this 
substance dissolved in 1000 ccs water. Since the molecular 
weight of potassium nitrate is ioi the solutions were respec¬ 
tively io*i, .i*oi, o*i, o*oi, o*ooi and o-oooi gm per cent. 
The solutions of the other substances were made isotonic 
with the six standards. As a consequence the solutions in 
any one vertical row of the Tables I, II, and III taken 
together are supposed to have an equal attraction 2 for water. 
In calculating the solutions required to be isotonic with 
the standards the isotonic coefficients of de Vries were em¬ 
ployed. De Vries 3 has shown that the attraction for water 
of any substance in solution depends partly upon the structure 
of the molecules. He proved by means of careful and in¬ 
genious experiments upon plant-cells that if the plasmolysing 
power of a molecule of potassium nitrate in a weak solution 
be supposed equal to 3, then the plasmolysing power per 
molecule of any other substance investigated (also in a weak 
solution) is approximately equal to either 2, 3, 4 or 5. De 
Vries has called these numbers isotonic coefficients. 
Substances in Table I, e. g. sodium chloride, have the 
isotonic coeff. 3, those in Table II, e. g. potassium sulphate, 
the coeff. 4, while for substances in Table III, e. g. cane- 
sugar, the coeff. is 2. This being so, it is obvious that to 
make solutions of sodium chloride, potassium sulphate and 
cane-sugar isotonic with a solution of potassium nitrate con¬ 
taining x molecules of the salt it is necessary to take f x mols 
sodium chloride, f x mols potassium sulphate, and f x mols 
cane-sugar. 
1 Note .—Instead of writing 1 gram-molecule in 1000 ccs the shorter expression 
1 mol will be subsequently employed in this paper. Cf. Grundriss der allgemeinen 
Chemie, Ostwald, 3. Aufl., p. 70. 
2 Expressed otherwise as osmotic force or plasmolysing power. 
8 Hugo de Vries, Eine Methode zur Analyse der Turgorkraft, Jahrb. fur wiss. 
Bot., Bd. xiv, 1884. 
