ANNIVERSARY ADDRESS, 11 
according to which the repulsion seen in the radiometer is — to 
internal movements of the molecules of the residual gas 
On the othiels hand, a French savant (M. Ledieu), in a paper 
read before the French Academy, remarks, that the theory which 
explains the action of the radiometer by saying that light falling 
on black dises becomes heat, and so establishes a difference in 
temperature between the dises and the gas in the case, which 
produces the motion, expressly requires that there shall never be 
an equilibrium of temperature between the discs and the gas in 
the case of the radiometer ; but this cannot be admitted, for the 
arms keep revolving at a uniform speed so long as the light is 
present. He had tried many experiments, and in one of them 
the instrument was heated nearly to redness, and the discs began 
to move, but the speed was sensibly accelerated by the momentary 
presence of a single flame, which joined its action to that of the 
radiant heat ; and he had obtained perfect rotation in an instru- 
ment in which both sides of the dise were equally polished. 
It will be remembered that in 1872 Herr Groneman, of Génin- - 
gen, propounded a new theory of the origin of the aurora. His 
hypothesis is that there are in space streams of minute particles 
of iron, revolving about the sun in the same way that meteors do, 
and that these, when passing the earth, become attracted to its 
poles, and from them stretch out as long filaments into 
but as they meet the earth’s atmosphere with planetary debit, 
they become ignited, and thus form the luminous aurora, giving 
with the spectroscope a greeniron line. He has recently returned 
to this theory, and brought forward much additional matter in its 
favour. In connection with this subject the researches of Pro- 
fessor Nordenskiold are very interesting. He has been examin- 
‘ing the purity of snow, both at Stockholm and near the North 
Pole. To north-west of Spitzbergen he found the snow con- 
taminated with minute black particles, which proved on examina- 
tion to be exactly the same as those found at Stockholm, and con- 
sisted of particles of metallic iron, phosphorus, cobalt, and rt 
ments of Diatomacee. ‘ 
