658 



Scientific Proceedings, Royal Dublin Society. 



be due to the presence of colloids, which, though raising the viscosity, at the 

 same time failed to retard the movement of the ion to a corresponding 

 extent. 



An experiment carried out with a gum arabic sol will make the meaning 

 of this clear. The conductivity of a decinormal solution of potassium chloride 

 in the sol is shown in Table V under C KCl ; C g represents the conductivity 

 shown by the sol, and G g + KCl that of the sol to which the salt was added. 

 The viscosities of the sol, one and four hours respectively after its prepara- 

 tion, are shown in the first two columns, and that after the addition of the 

 potassium chloride in the third column. If the viscosity of the sol after the 

 addition of the salt were inversely proportional to its conductivity, then the 

 application of the linear viscosity correction should raise the conductivity 

 shown by the potassium chloride in the presence of the colloid to 0-00715 

 [the conductivity of a decinormal aqueous solution of potassium chloride]. 



That this figure is greatly exceeded is shown in the last column, thus 

 demonstrating that the mobility of the ion has not been greatly retarded. 

 It will have been noticed that an increase took place in the viscosity of the 

 sol with age, and a decrease on the addition of the potassium chloride ; as 

 these are both characters shown by the majority of emulsoids, comment is 

 unnecessary. It is well to remember when considering the large viscosities 

 shown by the saps from Island Bridge that, unlike molecular solutions, great 

 changes in the viscosity of emulsoid sols [e.g., gum sols] may be brought about 

 by a very small increase in concentration- ; in these cases a considerable over- 

 correction may occur. Attention may be also drawn to the clumping of 

 colloids by metallic ions, which takes place in nearly all saps after extrac- 

 tion ; that electrolytes are concerned in this coagulation, and that it is the 

 gums which are mainly acted on is indicated by the continued precipitation 

 which takes place after killing the enzymes by boiling and filtering off the 

 coagulated albumen. 



An experiment recorded in Table VI will illustrate the insignificance of 

 albumen as a factor in raising the viscosity of the sap. The sap was pressed 

 from leaves taken from a tree growing at the Scalp. 



