12 
"stimulation efficiencies" partly because it is uncertain whe- 
ther — -, referred to above, is constant, and also because of 
the ion-proteid already present in the organism, the influence 
of which will be to lessen or to increase the effect of the 
testing re-agent. Still, the 'stimulation efficiency" of a re- 
agent will serve as a rough index of its probable eft'ect, and I 
therefore append a rough table of the re-agents most com- 
monly used as stimuli in j^hysiology, with their ionic veloci- 
ties and "stimulation efficiencies," the sign + before the 
stimulation efficiency denoting attraction of a "positive" 
organism, and the sign — attraction of a "negative" 
organism. 
If the stimulation efficiency be calculated from the ionic 
velocities it will not be accurate except for very dilute, com- 
pletely ionised, solutions. A more accurate method is to cal- 
culate the stimulation efficiency from the value of the trans- 
port number ?z, at the dilution which we are using. But, in 
order to make the table more general, I have, except in the 
cases of the carbonates and MgClg, calculated the stimula- 
tion efficiency from the ionic velocities. It is necessary to 
bear in mind, however, that solutions of a salt formed by 
the neutralisation of a strong base by a weak acid, as, for 
example, Na^COg, always contain OH ions, which have a 
very high velocity, and which tend to render the stimulation 
efficiency negative. Finally, in order to observe any propor- 
tion between the stimulation effects of different re-agents we 
must use equivalent solutions. The ionic velocities of 
Cu, Ba, Ca, SO^, and Ag, in the accompanying table, are 
taken from the results given by W. C. D. Whetham in the 
Philosophical Transactions of the Royal Society."^ Those of 
CI and I are from Kohlrausch's results, quoted by Whe- 
tham. f Whetham found that his results, obtained by a 
direct method, coriesponded very closely with Kohlrausch's. 
Those of K, Na, Li, H, NO,, *^ and OH are from Kohl- 
rausch's results quoted by Watson. J The stimulation effi- 
ciencies of Kg CO 3, Na^CO,, and MgCl.2 are calculated 
from the transport numbers for dilute solutions ('029, '093, 
and '087 equivalent gramme molecules per litre respectively) 
given in Fitzpatrick's "The Electro-Chemical Properties of 
Aqueous Solutions."^ 
* Vol. cIxxkIv. a, page 337: and vol. clxxxvi. A, pnge •507. 
t Ihid. 
I Textbook of Physics, 1900, poge 798. 
§ British As-sociiatioin Boport, 1893. Beprinted by Wlietliam 
in lii.s Theory of Solution and Electrolysis. 
