1900] CHANGE OF FORM IN GREEN ALGAE 297 
non-electrolytes. But the pressure of a simple solution of an 
electrolyte is not so easily calculated. On account of dissoci- 
ation, such pressure is much greater than that of a solution ofa 
non-electrolyte of the same gram-molecular concentration. It was 
this fact with which DeVries® was dealing when he derived his 
so-called isotonic coefficients. But since his range of concen- 
tration was very limited, he was unable to get at the truth of 
the matter; and the result is, that, though his coefficients may 
be, and doubtless are, true for a certain strength of solution, yet, 
Since dissociation itself varies with the concentration, they are 
fot true in general. However, in weak solutions dissociation 
becomes nearly complete, and if we assume that it is complete 
Weshall probably be able to approach the truth much more nearly 
than by following the method of DeVries. My solutions are all 
very weak, and I have no doubt that my approximations by this 
method are,as a rule, quite nearly true. By assuming complete 
dissociation, the calculated osmotic pressure for any solution 
becomes that of an equimolecular solution of a non-electrolyte, 
multiplied by the number of ions derived from a molecule of 
me salt under consideration. This makes the calculation very 
‘imple. Taking the pressure of one gram-molecule per liter of 
oo undissociated substance as a unit, W, table I gives the 
oo of the complete solution which was used a 
ee . his is practically the same as a so-called 2 per cent. 
: at as usually made up: In the pressure Yee 
Method - 4 assuming ionization complete; & by ee ae 
which om Z Bives: the actual pressures for a sa ee 
“ee : dissociation could be found. The geo ts 
te 8 6 lies almost wholly the Ca(NOs) a dia 
Su: € a greater error in using DeVries coefficien a 
spe complete dissociation. Column gives the num zi 
sce tived from a molecule, 7 is the ratio between the eS : 
: Pressure and that of an equimolecular solution 9 
“DeVairs, H Uco: 
Do Jahrbiicher fiir wiss. Bot. 14: 427. 1884. 
N 
PERS and HAMBURGER: Zeit. {. Physikal. Chemie 6: 319- 1890 
