REGULATION OF ENVIRONMENT 79 
was not diluted as a whole there was a very slight 
diminution in the proportion of salts to water. Some 
of the salts had presumably passed from the blood 
into the water contained in the intestine, with the 
result of decreasing very slightly the percentage of 
salts in the blood. The enormous extra secretion of 
water was the response of the kidney to this very 
slight change. At the end of the extra secretion the 
conductivity had returned to normal. 
When, instead of pure water, a dilute solution of 
sodium chloride in water was drunk, there was again 
an enormously increased secretion of urine. This was 
accompanied by an easily measurable dilution of the 
blood, and the slightly increased conductivity showed 
that not only water but also salt was in slight excess 
over the other constituents. Both water and salt pass 
out in the urine, though at first very little of the salt 
goes, indicating that the excretion of the extra water 
is a process independent of the excretion of the extra 
salt. 
After prolonged sweating, so as to deprive the body 
of much water, the urine becomes very scanty and 
concentrated. But in this case the blood may not 
become measurably more concentrated, even though 
the body has lost by sweating a quantity of water 
nearly equal in weight to the whole of the blood. 
The regulation of the proportion of water in the 
blood can thus be placed side by side as regards deli- 
cacy with the regulation of its reaction and compo- 
nents: its pressures of CO, and oxygen, its percentage 
of sugar, urea, salts, and albuminous substances. Had 
