HELMHOLTZ IN BERLIN 



of filter paper ; Hoffmann proved that a layer of cotton 

 wool had the same effect ; and Schroeder and Dusch 

 made the important demonstration, that' air filtered 

 through cotton wool is incapable of exciting putre- 

 faction in a putrescible fluid that has been boiled. 

 The cotton wool has sifted from the air the bodies 

 that cause putrefaction, just as the wall of the bladder, 

 the septum of filter paper, or the layer of cotton wool, 

 prevents the passage of the yeast cells from a fermenting 

 to a fermentable fluid. The conclusion, then, is irresist- 

 ible that the living organisms in the air and in the yeast 

 are the cause of putrefaction and fermentation. It will 

 thus be seen that the comparatively simple observa- 

 tions of Helmholtz were of fundamental importance. 



Surrounded as he was by young physicists, it is 

 not surprising that Helmholtz approached physiology 

 on its physical side. Physiology is essentially a com- 

 posite science, inasmuch as it is closely related to 

 anatomy, physics and chemistry. In the solution of 

 physiological questions, the physiologist must collect 

 facts from these three departments of knowledge. 

 Thus, for example, in investigating the phenomena of 

 the circulation of the blood, the physiologist, in the 

 first place, must be acquainted with the structure of 

 the heart and blood vessels, with the position and 

 appearances of the valves in the heart and in many of 

 the veins, and with the nature of the minute texture 

 or tissue of the heart, as revealed by the microscope. 

 Then he considers the circulation as a problem of 

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