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 



