18 
IOWA ACADEMY OF SCIENCES. 
example, mercuric chloride gives, in water, a freezing point 
depression nearly corresponding to its normal molecular weight, 
and the solution is a very poor conductor, while sodium chloride 
gives a depression corresponding to nearly half its normal 
molecular weight, showing a nearly complete dissociation into 
ions, and the solution is of high electric conductivity. 
The conception of an aqueous solution of such a substance as 
common salt, which presents it to the mind as consisting of sep- 
arated atoms of sodium and chlorine freely moving about in the 
water, is so contrary to preconceived notions upon which we 
were brought up that it is at first difficult to grasp. It must be 
remembered, however, that the ions, sodium and chlorine, are 
totally different things from the elements of the same name. 
The ions are atoms highly charged with electricity, which they 
lose as soon as they unite, to form molecules or separate in the 
elemental state. 
Moreover the ions, although free, are not independent. It is 
impossible, for example, to remove, as by diffusion or other- 
wise, from a solution any number of positive ions without 
removing a corresponding number of negative ions, because of 
the attraction due to the powerful opposite charges. The new 
theory explains clearly many things which were incomprehen- 
sible on the old theory of electrolysis. According to the old 
view, the electric current decomposed the electrolyte. Now, to 
do this, a certain assignable electro- motive force must be sup- 
posed to be necessary. To break any stated bond some definite 
force must be applied. But, in point of fact, the feeblest elec- 
tro-motive force will send a current through any electrolyte, and 
therefore will separate its constituents. There is no critical 
electro-motive force which must be applied to cause decomposi- 
tion. The new theory assumes that the electric current does 
not cause any decomposition at all, but simply moves through 
space the ions which had already been separated by the act of 
solution, and to do this the lowest assignable E. M. F. must 
suffice. 
The ratio of the conductivity of a solution to its concentra- 
tion increases with increasing dilution in such fashion so as to 
approach a maximum value for infinite dilution. This change 
in conductivity is a consequence of the fact that the degree of 
dissociation increases with increasing dilution, so that in the 
most highly dilute solutions, practically all the molecules of the 
dissolved substance are in the dissociated condition. The 
