Correspondence. 403 
to speak of the ws viva of an zmponderable body? But if 
zther be not supposed to be intermingled with palpable 
matter, then there is no object in attributing to it an ex- 
emption from the otherwise universal law of gravitation ; 
and it will then be imperceptible only because it exists 
beyond our reach. Moreover, as there are no means of 
limiting the possible amount of molecular displacement in 
a medium so attenuated, an amount of vzs viva is con- 
ceivable sufficient to impart effective motion to indefinitely 
denser matter; and thus this denizen of infinity may be 
assumed capable of executing its divine mission of impart- 
ing to material worlds those essentials to corporeal exist- 
ence, the very main-springs of organic life—light and 
heat. 
If these things are so, then (in American common par- 
lance) “the bottom falls out” of an experiment lately 
shown at the Royal Institution to demonstrate the pre- 
sence of zther by the heat developed in a metallic disk 
rotating in the exhausted receiver of an air-pump.* That 
the periodic retardation of Encke’s comet is due to the re- 
sistance of the universal medium is highly probable; but 
this experiment, if it proves anything in the same direction, 
froves a great deal too much; for if such be the viscisity 
of the medium hypothetically present in the receiver, that 
the addition of a notable quantity of air (5 or 10 per cent. 
as was stated), makes no sensible difference in the heat 
generated by friction, it is difficult to conceive how any of 
us have hitherto escaped resolution into our gaseous ele- 
ments. On the contrary, when our earth and its envelope 
enters a probable atmosphere of orbitating fragments (of 
which we have recently had such a magnificint experience), 
some of these crumbs of the universe which have for an 
indefinite period harmlessly traversed the ethereal medium 
of infinite space, at the enormous velocity of perhaps 30 
or 40 miles in a second, become immediately ignited, 
and probably consumed by friction in the confines of our 
atmosphere, which must there be attenuated to a degree 
never yet attained, except, perhaps, in the vacuum-tubes 
of Mr. Gassiot. 
Permit me, in conclusion, to repudiate (needlessly 
perhaps) any claim to originality in the general idea that 
* Proceedings of the Royal Institution, vol. iv., p. 563. The 
writer has since learnt that the ether-friction theory has been 
judiciously withdrawn by its author for further consideration. 
