VELOCITY OF REACTION. HETEROGENEOUS SYSTEMS 135 



finely powdered coagula of egg-white or the emulsified fat 

 drops in Volhard's, Stade's, and Engel's experiments. But 

 with the pillars of coagula this cannot be said to occur. In 

 this case we must suppose that the liquefied part of the 

 column is carried away into the liquid about, so that this is 

 able to attack new portions of the pillar. Here, therefore, 

 the reaction is going on at the surface of the coagulated 

 substrate, and immediately below it, and the constancy of 

 the product qt shows that under constant conditions the 

 number of molecules digested in one second is proportional 

 to the concentration of the ferment in the surrounding 

 liquid (and to the surface, which remains nearly constant). 

 Madsen and Walbum have investigated the progress of 

 dissolution of casein by means of trypsin. Ten g. of casein 

 in the form of the powder were suspended in 100 c.c. of 

 a i per cent solution of trypsin and held at constant tem- 

 perature. In order to prevent the casein from subsiding, 

 the flask containing the mixture was steadily shaken. 

 After different time intervals samples were removed, and 

 the undissolved quantity of casein determined by means of 

 its content of nitrogen determined according to the 

 method of Kjeldahl. The progress of dissolution is shown 

 by the following table (temperature, 34.1) : 



