Mr. M. M. Pattison Muir on Gallium. 281 



cave to the convex side. Thus it appears that the motion of 

 the radiometer due to the hot shade or hot water is opposed in 

 direction to that caused by the light. This result accords 

 with the experiment mentioned at the top of p. 314 of the 

 ' Proceedings.' 



The most essential part of the theory proposed above is the 

 dynamical action of a circulating aetherial current. From 

 hydrodynamics we know that such a current could not begin to 

 flow in a perfect vacuum, and that it flows with more difficulty 

 in proportion as the vacuum is more complete. This is exem- 

 plified by the difficulty of producing a galvanic discharge in 

 very rare air between distant poles. The proposed theory, 

 therefore, gives a sufficient reason for the decrement of the 

 action of the radiometer as the exhaustion of the air advances 

 towards completion. From this point of view I am wholly 

 unable to admit that the presence of residual gas is the cause 

 of the movement of the radiometer. It seems to me that one 

 might as well argue that because a bell is not heard when rung 

 in an exhausted receiver, the air in the receiver, and not the 

 tongue of the bell, produces the sound when it is heard. 



I take occasion to say here, as I have said before, that I 

 have been induced to give particular attention to theoretical 

 explanations of phenomena of the radiometer solely on account 

 of the adaptability of such explanations to elucidate the hy- 

 drodynamical theory of the physical forces. 



Cambridge, March 13, 1877. 



XXXVIII. Observations on Gallium. By M. M. Pattison 

 M.vtk,F.R.S.E., Assistant-Lecturer on Chemistry, the Owens 

 College, Manchester, 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 ■ TN your Magazine for 1876 you published a translation, by 

 JL me, of a short paper written by M. Mendelejeff which ori- 

 ginally appeared in vol. lxxxi. of the Comptes Rendus, p. 969. 

 In this paper Mendelejeff drew attention to the " periodic law " 

 announced by him in 1869, according to which " the proper- 

 ties of the elementary bodies, as also the properties and con- 

 stitution of their compounds, are periodic functions of the ato- 

 mic weights of the elements." In accordance with this hypo- 

 thesis, the elements may be arranged in certain groups ; 

 remarkable relations are shown to exist between the proper- 

 ties of the members of each group. The third group com- 



