IOWA ACADEMY OF SCIENCES. 
43 
a hydrogel. Protoplasm, moreover, presents the most com- 
plete reversibility of the process, the sol passing into the 
gel and the gel back into the sol again, so long as condi- 
tions for vital phenomena persist. The importance of this 
fact in the life of the cell has not as yet been fully grasped 
by biologists generally. 
No less fundamental is the recent brilliant work of 
Hardy on the conditions which are productive of gelation 
in colloidal solutions. A sol of proteid was used, and it 
was made clear that the gel state is produced by the addi- 
tion of electrolytes but not by non-electrolytes, unless 
these act chemically; and that the gelation from electro- 
lytes is due to the electric charges carried by the ions was 
demonstrated by the identical results following the use of 
an electric current from a battery. Moreover, the signs of 
the electric charges carried by the ions, — positive or nega- 
tive, — determine the movements of the colloidal particles, 
keeping them in suspension as a sol, or causing them to 
fall into the gel condition. For example, a sol having its 
colloidal particles negatively charged will pass into the gel 
state upon the addition of positive ions, or the introduction 
of the positive electrode of a battery. 
These remarkable results indicate the importance of 
ions in the life of the cell, and they also serve to clarify 
many phenomena hitherto quite obscure. The rapid 
changes in the consistence of protoplasm have long been 
known from observation, but they have never been 
explained; these fascinating alternations of more rigid and 
more fluid conditions are now seen to rest upon the state 
of the colloids of the cell, the controlling factor being the 
ions set free as the result of chemical reactions. Pecul- 
iarities in the toxic effects upon protoplasm of certain 
substances have long been a puzzle; but such effects are 
now recognized as due to the electric charges carried by 
the ions rather than to the chemical nature of any particu- 
lar substance employed. 
