78 ORGANISM AND ENVIRONMENT 
as is usually passed in twenty-four hours. The urine 
consists of nearly pure water, containing only what 
are relatively speaking traces of the ordinary urinary 
constituents. Now this fact in itself is very remark- 
able. The blood plasma contains a considerable 
amount of sodium chloride, and usually there is more 
sodium chloride in the urine than in the blood plasma; 
but in the urine secreted after water drinking there is 
hardly any sodium chloride. The sodium chloride is 
held back, while the water passes in large quantities. 
What we wished, however, to investigate specially 
was the change in the blood to which the increased 
secretion was a response. One would naturally look 
for evidence of dilution of the blood by the water ; and 
dilution would be shown by a diminution in the 
percentage of haemoglobin, since this can be measured 
with great accuracy and none of the haemoglobin is 
excreted or destroyed. There was, however, no 
diminution in the haemoglobin percentage during the 
period of most rapid excretion of the urine. Evi- 
dently the blood was not diluted, in spite of the fact 
that sometimes a volume of liquid exceeding that of 
the whole of the blood had been carried by the blood 
from the intestines to the kidneys in the course of 
a few hours. 
Dr. Priestley then determined the electric con- 
ductivity of the blood serum, as this gives a very 
sensitive measure of the concentration of salts in the 
blood. The result was that there was a very slight 
but constant diminution of the conductivity during the 
extra secretion. This proved that though the blood 
