COAGULATION OF THE BLOOD 109 



way, I found in general that after being at rest for ten 

 minutes, the blood continued fluid ; nay, that after being at 

 rest for three hours and a quarter, above two-thirds of it 

 were still fluid, though it coagulated afterwards. Now the 

 blood when taken from a vein of the same animal was 

 completely jellied in about seven minutes. The coagulation 

 - of the blood in the basin and of that which is at rest are so 

 different that rest alone cannot be supposed to be the cause 

 of the coagulation out of the body." This is not clearly 

 expressed, but it evidently means that were rest the sole 

 cause of coagulation the blood at rest in the vein would 

 have coagulated as quickly as the blood in the basin. We 

 cannot follow Hewson further in his investigations. He cuts 

 out the ligatured vein and freezes it and shows that after it 

 has been thawed it is still fluid and still ready to coagulate. 

 He places the excised vein in water, which he warms to 

 various temperatures, and finds that it is not immediately 

 coagulated at 114 Fahr., although it is at 120 Fahr. And 

 lest this result should be regarded as a heat-coagulation, 

 such as occurs when a solution of white of egg is heated, 

 and not the natural process, ' ' It may be necessary to ob- 

 serve here that the part coagulated was only the lymph 

 (plasma) ; for the serum requires a much greater heat to fix 

 it, that is, a heat of 160, as will appear hereafter." Hew- 

 son's methods closely resemble those of his contemporaries 

 William and John Hunter, Henry Cavendish, Antoine 

 Laurent Lavoisier. We have given these few extracts 

 from Hewson's book in his own words because they show 

 how thoroughly he was embued with the great principle 

 which may be said to have dawned upon Science at this 

 period, supplying a code of rules to the observance of 

 which all subsequent advance was due the principle of 

 the control experiment. He arranged that only one of the 

 suspected causes should act at a time, and he had the 

 scientific insight which warned him that one experiment 

 under natural conditions was better than a hundred in 



