Intelligence and Miscellaneous Articles. 233 



in the tube, and the globes were otherwise sealed up. Any light 

 which enters through the clean faces is received on the black and 

 white surfaces ; and the air in the globes expands in accordance 

 with the difference of temperature which they attain, moving the 

 oil in the tube. A motion of h an inch on the part of the oil shows 

 a difference of 2°'2 in the temperature of the air within the globes. 



The instrument so constructed is exceedingly delicate, and will 

 show a difference in the intensity of light sufficient to make one 

 revolution per minute difference in the speed of the mill. As a 

 photometer it is much more convenient than the mill ; and its con- 

 struction presents much less difficulty. By making the lower por- 

 tion of the siphon-tube horizontal, and usiug glass indices after 

 the manner of Rutherford's thermometer, the instrument might be 

 made to record maxima and minima intensities of light, as well as 

 be more delicate in other respects. 



Measured with this instrument, the light necessary to give the 

 mill 240 revolutions per mi n ute does not exceed 24°, and is pro- 

 bably less than this, which shows that the theoretical difference of 

 heat necessary to cause the heat-reactions is well within the differ- 

 ence as actually measured, leaving an ample margin for error in 

 the methods of approximation used in the calculation. 



In concludiug the paper the author claims to have set at rest the 

 only point respecting the explanation of the motion caused by 

 heat which remained doubtful after he had discovered that, accord- 

 ing to the kinetic theory, the communication of heat to a gas must 

 cause a force reactionary on the surface, viz. whether this reaction 

 was adequate in amount to cause the results seen to take place. 



He adds a suggestion as to a new form of light-mill to have 

 vanes inclined like the sails of a windmill, and not having one side 

 white and the other black, like the light-mills at present construc- 

 ted, arguing that the forces act perpendicularly to the surface, and 

 in a direction independent of that from which the light comes ; so 

 that such a mill would turn like a windmill with the full and not 

 merely the differential effect of the light. Such a mill, he con- 

 cludes, would furnish another test as to whether or not the force is 

 directly referable to radiation. 



XXXI. Intelligence and Miscellaneous Articles. 



ON THE ELECTROMAGNETIC ACTION OF ELECTRIC CONVECTION. 

 BY DR. HELMHOLTZ *. 



j" UXDERST AXD by electric convection the conveyance of electri- 

 •*- city by the motion of its ponderable bearers. In my last memoir on 

 the theory of electrodynamics t, I proposed some experiments 

 (which were then carried out by Herr X. Schiller) in which the 

 question came into consideration whether electric convection is 

 dynamically equivalent to the flow of electricity in a conductor, as 



* A Report on some experiments carried out by Mr. Henry A. Row- 

 land, of J. Hopkins's University in Baltimore. 



t Monatsbericht of the Berlin Academy of Sciences, June 17, 1375, 

 p. 405. 



