TRANSACTIONS OP SECTION I. 687 



similar to the iioinial, but in unfavourable conditions matclies similar to a 

 dichromic, and with small and dim lights he will confuse red with green. It is, 

 however, easy to show that he is colour-blind in ordinary circumstances. When 

 examined with my spectrometer he will declare that there are only three colours iu 

 the spectrum — red, green, and violet^and mark out the spectrum into about ten 

 instead of eighteen monochromatic divisions. The pseudo-isochromatic tests 

 show how defective his hue differentiation is as compared with the normal and 

 that the defect is not limited to the fovea. Diminished hue discrimination 

 accounts for these cases, the trichromic behaving in ordinary circumstances as a 

 normal-sighted person in difficult circumstances. Even in those cases in which 

 there is defective light-perception, as for instance shortening of the red end of 

 the spectrum, this cannot be accounted for by a diminished red sensation, whicli 

 is affected by other parts of the sjJectrum, as the shortening may be such that 

 any amount of red light from the shortened region may be added to an equation 

 without altering its appearance to the colour-blind. 



7. The Effect of Removal of ilie Post-cenlral Uyri on tJie Moveinenta 

 of the Anthropoid Ape. By Dr. T. Graham Brown. 



8. The Effect of Thyroklectoiiiij and Thyroid-feediiuj on the Adrenin 

 Content of the Supra-renals. By Professor P. T. Herring. 



THURSDAY, SEPTEMBER 9. 



The following Papers were read : — 



1. The Mode of Action of Urease. 

 By Professor W. M. Bayliss, F.R.S. 



Urease is active in solutions in which it is insoluble — strong alcohol, for 

 example. It, therefore, acts at its surface, and, presumably, by condensing 

 urea on this surface, and increasing the rate of reaction with water by mass 

 action, or perhaps by the intervention of molecular forces in the act of 

 condensation. 



This fact explains the eifect of various substances on the rate of the reaction. 

 This action may be of two kinds, one in changing the degree of colloidal dis- 

 persion and by this means the active surface, the other by taking possession of 

 the surface and displacing the urea therefrom. Electrolytes have the former 

 action, which may be either in increasing the dispersion and accelerating the 

 reaction, or in aggregation and incipient precipitation, which results in retarda- 

 tion by decrease of the active surface. The former effect is shown by weak 

 acid or phosphate. The latter is shown by multivalent ions, such as lanthanum. 



The effect due to displacement of urea from the surface of the enzyme is 

 shown by the so-called ' sui-face-active ' substances, such as amyl alcohol, 

 saponin, bile salts, &c. Since this effect is due to depression of surface energy, 

 which has a negative temperature coefhcient, it is interesting to find that the 

 retardation in question is greater at low than at higher temperatures. 



Another fact which is explained by the adsorption of urea by the surface of 

 the enzyme is the exponentiaf ratio between the concentration of the enzyme 

 and its activity, together with the constancy of the rate of reaction above a 

 certain concentration of the substrate. This last fact is due to the saturation 

 of the surface, a very common circumstance in adsorption. 



Concentrated solutions of urea have a remarkable retarding action, not only 

 on urease, but on other enzymes. It is not to be explained by viscosity, since 

 more viscous solutions of saccliarose have not nearly so great an effect. This 

 fact seems also to exclude the removal of water as an explanation. It is 

 probably due to some action related to the solvent properties of urea to which 

 Ramsden has called attention. 



