or positive ions in different places in the tissue, or, in other 
words, to differences in concentration of the ions." * 
These are the only important allusions to tneories similar 
to mine which I have been able to find ; but, as the literature 
to which I have access is limited, my apologies are due to any 
authors whose published theories I may have put forward as 
original. 
I do not, by any means, regard the whole of the hypo- 
theses and deductions put forward in this paper as proved. 
Indeed, this paper is rather to be looked upon as providing 
an outline to be in the future corrected and filled in by an 
extended series of experimental investigations. My theory 
of chemotaxis, put forward in section 3, and some of my 
views on the propagation of excitation in muscle, put forward 
in section 6, are, however, upon a somewhat different footing, 
inasmuch as they already receive strong support from the 
experiments described in these sections, on infusoria, on the 
one hand, and on the intestine of a fly, on the other. I maj 
state that I am about to bring forward strong experimental 
evidence in support of my views in section 13 of this paper on 
rhythmicity in muscle, and, at the same time, of those in 
sections 6 and 7, on the influence of the mass of ions upon the 
formation of ion-proteids in excitable tissues. I also hope 
before long to publish further experimental evidence touching 
my views on the transmission of excitation, and also further 
experiments on chemotaxis. 
In concluding these introductory remarks, I desire to 
express my gratitude to Professor E. C. Stirling, F.R.S., for 
his suggestions, for facilities afforded me for experiments, and 
for the interest which he has taken in the preparation of this 
paper, and in the experiments; to Dr. C. J. Martin, F.R.S., 
and to Mr. J. A. Craw, for the care with which they read the 
paper and for their criticisms ; to Professor W. H. Bragg, for 
a valuable criticism ; and to Mr. W. Fuller for his advice and 
practical assistance in some of the experiments. This paper 
was written nearly a year ago, but, owing to its having been 
put into the hands of others, at a distance, for their considera- 
tion, its publication has been delayed. 
i- — Contact Difference of Potential between Electro- 
lytes AND ITS Influence upon Surface Tension. 
It is a well-known fact that when two electrolytes, or two 
solutions of different concentration of the same electrolyte, 
are in contact, there is a difference of electric potential 
between their bounding surfaces, just as there is a difference 
* American Jounial of Physiology, vol. xi., No. 5, page 457. 
