625 
ON RADIATION AND ABSORPTION, WTTH REPBRENCE TO THE COLOUR 
OF BODIES AND THEIR STATE OF AGGREGATION. 
BY JOHN TYNDALL, ESQ., LL.D., F.R.S. 
Delivered at the Royal Institution , Friday, January 19, 1866. 
The speaker referred to the relation subsisting between the sensible phenomena of 
nature, and those processes lying beyond the range of the senses, on which- the pae 
mena immediately depend. He spoke of the function of the imagination mpictan 
operations which, though great in their aggregate results beyond ajd 
minute individually to be capable of oDservation. He referred to the luminiferous etha 
that fills space, as the most striking illustration hitherto known of .^ e . P r0 ^fXmed 
line of thought from the domain of the senses into that of the imagination and affirme 
the existence of this wonderful medium to be based upon proofs at least as strong as 
those which sustain the theory of gravitation. , i wtnVb are 
Dwelling briefly on the relation of this ether to the atoms and molecules which are 
plunged in it, he illustrated, by reference to the phenomena of sound, the dim* 
between good and bad radiators. A naked tuning-fork vibrating in free air imparte . 
small an amount of motion to the air that it ceased to be heard as soundat an incon¬ 
siderable distance; the same tuning-fork brought into union with ‘‘V^fferWas a bad 
duced a sound which could be heard by thousands at ounce. The F“^b,Nation of the 
radiator, the combined fork and case was a powerful radiator. ! 
fork and its case, as regards sound, roughly represented the influence ofc ™“,“, 
bination as regards radiant heat. By the act of combination the power of the ooml n n 
atoms as radiators might be augmented ten-thousandfold. As an example.of this tl. c 
vapour of water was selected; and it was affirmed that a pound of this vapour take 
the ton of a high mountain, there heated and exposed before the cloudless heaven, would 
radiate nine X thousand times-possibly twenty thousand 
stellar space as could be radiated by either of the constituents of the vapour whm u 
combined aker ^ referred to the well-known analogy between the pitch of a sound 
and the colour of light, and throwing a large spectrum upon a white> screen mentioned 
the relation between the various colours to the rapidity <> f ethowalvxbr^on. Thespwj 
from the red to the blue embraced an infinite number of rates of f 
and continuously shortening without any interruption. » be “ £ 
finite number of tuning-forks of gradually augmenting pitch, and all sounding at the 
same time This spectrum was derived from the carbon points of the electric light; but 
it was shown that in the case of various other incandescent substances the spectrum wa 
no? f this coXIous character. The magnificent stream of 
the volatilization of silver in the elect™ lamp was.shown. upon ai screenand.afterwards 
the lip-ht was analvzed and found to produce two bands of brilliant green, ameimg d 
digX from eaTother in refrangibifity. Here the case is typified ~ by an mfimte 
number of tuning-forks, but by two tuning-forks of slightly different-pitch- And just 
as the rate of vibration in the case of the tuning-fork is a fixed rate so th irate of b 
tinn of the atoms of silver vapour were fixed. And as the coloui of the vapour dope 
on its rate of atomic vibration, the constancy of this rate secured the constancy of co oui 
n ^vlur We cannot mike the vapour of silver white hot , however we may exalt 
its temperature We may augment the brilliancy of the particular rays that it emits, 
butw/cannot cause it to emit that variety of rays the blending of which together pro- 
ll TTkftheXur°of "silver tlie vapour of water has also its definite periods of vibration; 
body which cannot emit luminous rays is incompetent to absorb them, ihus. the s 
luminous rays pass freely through the aqueous vapour of our atmosphere, whi e t 
