3 8o SCIENCE PROGRESS 



crystals of lead formate were subsequently identified under 

 the microscope. It is of course possible, even now, that the 

 formic acid may have been produced during the distillation 

 of the original extract with phosphoric acid. In any case, the 

 author points out that the establishment of formic acid in no 

 way affects the question as to whether or not this acid is the 

 main cause of the intense irritation produced by stinging- 

 nettles. According to Haberlandt (1886) the active poison is 

 most probably an enzyme. 



The mode of action of colloidal silver sols has recently 

 been studied by Professor Marshall (Proc. Roy. Soc. Edin., 

 1919, 39 (2), 143). He finds that the bactericidal action of 

 silver sols is relatively slight as compared with that of the 

 free silver ions in a silver nitrate solution. Such bactericidal 

 action as they have is, however, not due to bombardment by 

 particles of silver in Brownian movement, since bacteria were 

 seen to move freely for long periods in a moderately concen- 

 trated colloidal metal sol, in spite of their being frequently 

 bombarded. The chief antiseptic action of silver sols appears 

 to be due to ultramicroscopic particles between 5 [ip and 

 15 yu/i diameter, and it is suggested that these particles may 

 be taken up by the bacteria and converted into soluble pro- 

 ducts within the organism. This would explain why colloidal 

 silver sols act more slowly than ionised silver. 



Bancroft (/. Phys. Chem., 1919, 23, 356 and 365), in the 

 course of his studies on the colours of colloids, discusses the 

 blue colour of eyes and feathers. With regard to the former 

 it is pointed out that there is no pigment on the front of the 

 iris in blue eyes ; the colour is due to turbid media, and is 

 deeper the finer the suspended particles. Other colours in eyes 

 are due to pigments in the front of the eye which thus mask the 

 blue colour produced by the turbid media. In the case of 

 feathers the colours fall into three categories : firstly, those 

 which are produced by a pigment, and include black, brown, 

 orange and yellow ; secondly, blue and violet, which are struc- 

 tural colours produced by finely divided air-bubbles contained 

 in a transparent layer below the epidermal cells ; thirdly, there 

 are colours depending on the position of the light and the eye 

 produced by a sheath acting like a prism. By displacing the 

 air-bubbles in the case of the blue feathers by a liquid such as 

 Canada balsam or benzene, which has the same refractive index 

 as the cell-wall, the blue colour disappears. 



GEOLOGY. By G. W. Tyrrell, A.R.C.Sc, F.G.S., University, Glasgow. 



Economic Geology. — F. L. Stillwell has continued his researches 

 on the origin of the gold-quartz veins of Bendigo in " Factors 



