Royal Society. 7 3 



it may cross in succession the planes of polarization of the different 

 coloured rays, and so produce the most vivid effects. The dispersion 

 of the brushes by a plate of quartz, however, may be studied by em- 

 ploying an additional polarizer and quartz plate between the source 

 £>f light and the whole system previously used. By turning this 

 polarizer round we extinguish each ray of the spectrum in turn and 

 tint the whole field with the complementary colour. The brushes 

 will then appear to revolve about their centres as the tints vary 

 continuously from one end of the spectrum to the other. If the 

 polarizer be turned still further round, the tints which had changed 

 continuosly from red to violet, or vice versa, change suddenly from 

 violet to red, or vice versd, and the brushes jump suddenly back to 

 their original position. 



This last optical arrangement may be employed to examine the 

 more important phenomena of the dispersion of the optic axes 

 produced, not by a quartz plate between the usual polarizer and 

 crystal, but by certain biaxial crystals themselves. 



ROYAL SOCIETY. 



[Continued from vol. xliii. p. 542.] 



Marcli 14, 1872.— The Earl of Kosse, D.C.L., Vice-President, in the 



Chair. 



The following communication was read : — 



" The Decomposition of Water by Zinc in conjunction with a more 

 Negative Metal." By J. H. Gladstone, Ph.D., F.R.S., and Alfred 

 Tribe, F.C.S. 



Pure zinc is incapable of decomposing pure water, even at 100° C, 

 but at a considerably higher temperature it is known to combine 

 with its oxygen. Davy exposed pure water for two days to the action 

 of a pile of silver and zinc plates, separated only by pasteboard, with- 

 out obtaining any hydrogen ; Buff, however, has shov/n that a very 

 minute trace of gas can be formed at the ordinary temperature by 

 a pair of zinc and platinum plates. 



During a series of experiments, of which we have already published 

 an instalment, it occurred to us to ascertain whether by bringing the 

 two metals closer together, and so increasing the electrical tension of 

 the liquid, we could effect the same combination of zinc with oxygen 

 at the ordinary temperature which takes place without the second 

 metal at a very high temperature. Thin sheets of zinc and copper 

 were hammered together and placed in a bottle filled with distilled 

 water. Small bubbles of gas were formed. The experiment, however, 

 was tried in a more perfect form. Some zinc-foil was allowed to 

 remain in a somewhat dilute solution of copper sulphate until its 

 surface was well covered with spongy copper. The metals were 

 thoroughly washed with distilled water, and then they were immersed 

 in a bottle of distilled water with a delivery-tube. Minute bubbles 

 of gas quickly made their appearance, which proved to be hydrogen, 

 and zinc-oxide was formed. Two eNperiinents were made quantita- 



