must decrease the surface tension, whichever side is positive. 
Hence, by applying an external E M F, so as to make the 
mercury negative, and increasing it till the surface ten- 
sion, as indicated by the pressure which has to be applied to 
bring the meniscus to its sighted position, is a maximum, will 
be exactly equal and opposite to the contact difference of 
potential between the mercury and the sulphuric acid solu- 
tion. In this way Lippmann found that the contact differ- 
ence of potential between mercury and sulphuric acid solution 
was about 1 volt."* 
2. — The Ion-Proteid Theory. 
This theory, due to Loeb, is that when an ionised electro- 
lyte diffuses into protoplasm the ions after this diffusion do 
not remain dissociated, but that they enter into loose combi- 
nation with some proteid constituent of the protoplasm, this 
compound being known as ion-proteid. Loeb has brought 
forward many facts in support of this view, t which we need 
not enter into here, as we shall find many even more cogent 
reasons for adopting it in the sequel. I will only quote, after 
Loeb, a statement made by Dr. W. Pauli, of Vienna: — Wa 
cannot doubt the general existence of ion-proteid compounds 
in the living organism. We have even urgent reasons for 
assuming that all the proteids of the protoplasm exist there 
only in combination with ions." Thus it would appear that 
the bulk of protoplasm is formed of ion-proteid compounds, 
and, indeed, it seems probable that they represent the culmi- 
nating point of anabolism. We shall see the reasons for this 
view later. 
If this be true, then it follows that, owing to metabolism 
and to dissociation analogous to the dissociation into ions of 
electrolytes, a number of these ions must, in general, exist in 
the protoplasm in a dissociated state, so that there will, in 
general, be a contact difference of potential between any proto- 
plasmic body and tlie (liquid) medium in which it is sus- 
pended. This has been directly proved by W. B. Hardy :|: in 
the case of particles of albumin suspended in acid and alka- 
line solutions. He states his conclusions thus: — ''The proteid 
particles, therefore, have this interesting property : that their 
electrical characters are conferred upon them by the nature 
of the re-action, acid or alkaline, of the fluid. If the latter 
* W«.t«oii : Textbook of Physics,, 1900, page 814. 
t \'ide On lon-pi-oteid Compounds and their role in the 
Meclianios of Life Phenom-ena. American Journal of Plivsiolosrv, 
1900. 
I On the Coiagulation of Proteid bv Electricity. Journal 
of Physiology. June, 1899. 
