28 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



characteristic odor of coniine and nicotine are well known, as is 

 also the very bitter taste of sparteine. With the exception of 

 thebaine, the other alkaloids mentioned also have pronounced 

 bitter testes. The taste of ecgonine, which, though very solu- 

 ble, passes through membranes slowly, has been described as 

 bitter-sweet. Thebaine seems to behave in an exceptional man- 

 ner; it is very soluble, veiy poisonous, passes through mem- 

 branes with great readiness, and yet is tasteless according to 

 some authorities. 1 I have not been able to verify this since a 

 sample of the substance was not available. With the apparent 

 exception of thebaine, however, it is important to note that the 

 alkaloids which diffuse readilv through membranes and which 

 are known to exercise a strong physiological effect on the nerves 

 are also able to get at the nerve endings of the sense of taste, re- 

 acting upon the same with vigor, producing disagreeable burn- 

 ing or bitter tastes. 



Colloidal Solutions. 



As typical colloidal solutions may be mentioned solutions of 

 dialyzed silicic acid, ferric hydroxide, aluminum hydroxide, also 

 solutions of albumen, gelatine, gums, etc. These solutions are 

 practically non-conductors of electricity, i. e., they contain few 

 or no ions; they have boiling and freezing points that differ but 

 very little from those of water. The rate of diffusion of colloids 

 is very slow, and in general they are very inert in their chem- 

 ical behavior. Besides those already mentioned, other charac- 

 teristics of colloidal substances are that they have no definite 

 solubility and that their solutions gelatinize when treated with 

 certain reagents. These colloidal solutions are devoid of taste. 

 This is very likely due to the fact that the molecular weight of 

 colloidal substances is very large and their diffusion so very 

 slow that they can not get at the nerve endings; though, be- 

 cause of their inert character, it is quite reasonable to sup- 

 pose that, even if they were able to come into immediate contact 



1 See for example Watt's Dictionary of Chemistry 4, p. 68L 



