354 Scientific Proceedings, Royal Dublin Society. 



attributed to the clumpiug together of the particles of colloid into aggregates, 

 separated from one another by liquid. This action is plainly visible in 

 the later stages. 



It is well known tliat electrolytes have the power of precipitating colloids ; 

 and that the clumping of the gel, which is formed by the action of pectase on 

 pectin, is probably due to the action of electrolytes is shown by the fact that 

 a slight increase in the quantity of electrolytes present has greatly increased 

 the rapidity of clumping. An experiment which will be described later 

 appears to afford conclusive evidence in favour of this view. 



Experiment V. 



In the last two experiments the rate of increase in viscosity was consider- 

 ably less than in Experiment II. The optimum temperature for the action of 

 pectase is stated by Fremy to be 30° 0., and, therefore, one would expect that 

 at 21° 0. the action would have been more rapid than at 14° 0. That the 

 opposite was the case seems to show tliat the sap was less active in the later 

 experiments, or else that the pectin solution had become weaker through decom- 

 position. The sap used in Experiment II was extracted on the 14th of May, 

 soon after the leaves had emerged from the bud, whereas that used in 

 Experiments III and lY was extracted on the 22nd June. 



If the sap iu the newly formed leaves contained more pectase, it is probable 

 that if only young leaves were used the sap would be more active than that 

 obtained chiefly from full-grown leaves, as was usually the case. On the 

 22nd of June sap was extracted from young leaves which were less than one- 

 third the size of the full-grown leaf. An experiment was performed in the 

 usual way at 21° C, but the rate of increase in viscosity was found to be 

 about the same as in Experiments III and IV. To avoid confusion a curve 

 was not plotted, as this experiment was carried on for two hours only. The 

 result seems to show that tlie slower action in these experiments was not due 

 to a decrease in the amount of pectase present, but was due to partial 

 decomposition of the pectin solution. It is probable, however, that further 

 experimental work would sliow that there is a variation in the amount of 

 pectase present in the sap of leaves of different ages. 



Experiment VI. 



In Experiment IV an addition of electrolytes greatly increased the rate 

 of clumping of the gel. In this experiment the object iu view was to remove 

 the electrolytes as far as possible, and see what effect this would have on the 

 clumping. 



