114 



COLLOIDS IN BIOLOGY AND MEDICINE 



/Thermometer 



then a constriction as well as markings. This tube is placed in a 

 large glass shell which can revolve on its axis and has a thermometer 

 inserted. Since the apparatus may be turned upside down like an 

 hour glass, it is possible to take several successive readings from the 

 same quantity of fluid. DETERMAN employs it chiefly for the de- 

 termination of viscosity in uncoagulated blood. For this purpose 



he places a trace of hirudin on 

 the unbroken skin, preferably on 

 the lobe of the ear. After punc- 

 turing the skin he collects the 

 blood with a pipette directly 

 connected with the tube of the 

 viscosimeter. 



The apparatus of W. HESS* 

 depends upon a somewhat differ- 

 ent principle. He does not com- 

 pare the time of flow, but the 

 distances fluids may be sucked 

 up. His apparatus consists of 

 two capillaries connected with a 

 T-tube, through which fluids are 

 sucked with a rubber bulb; 

 through one capillary water is 

 sucked, and through the other 

 blood or some other fluid that is 

 to be investigated. From the 

 ratio between the distances to 



which the two fluids are sucked through the capillaries, the viscosity 

 may be directly determined. The apparatus has certain special 

 advantages; the horizontal position of the capillaries eliminates the 

 influence of the specific gravity; and since water and colloid are 

 simultaneously tested, the errors of temperature are reduced to a 

 minimum and calculations for correction are unnecessary. 



FIG. 24. Viscosimeter, (H. A. 

 Determan.) 



Melting, Coagulation and Solidification Temperatures. 



The determination of the melting, 1 coagulation and solidification 

 temperatures has the same significance for colloids as the measure- 

 ment of the melting point has for crystalloids. 



Coagulation by Heat. The fluid to be investigated is placed in 

 a test tube, in a water bath. The contents of both test tube and 



1 In the case of jellies it is only possible to speak of a " period of liquefaction "; 

 for the sake of simplicity I employ the expression "melting point." 



