334 
PEOFESSOE TYNDALL ON THE ACTION OF BAYS 
clianically together in the proportion of 14 : 3 ; the amount of radiant heat which they 
then absorb is augmented more than a thousandfold * the moment they build themselves 
together into molecules of ammonia. Neither the quantity nor the ultimate quality 
of the matter is here changed ; the act of chemical union is the sole cause of the enor- 
mous alteration in the amount of heat intercepted. The converse of these statements 
is of course also true ; dissolve the chemical bond, either of the nitrous oxide or of the 
ammonia, and you instantly destroy the absorption. As a proof that our atmosphere is 
a mixture, and not a compound, no experiment with which I am acquainted matches in 
point of conclusiveness that which demonstrates the deportment of dry air to radiant heat. 
But the molecules which can thus intercept the waves of ether must be shaken by 
those waves, possibly shaken asunder. That ordinary thermometric heat provokes 
chemical actions is one of the commonest facts of observation. These actions, considered 
from a physical point of view, are changes of molecular position and arrangement con- 
sequent on the acceptance of motion from the source of heat. Badiant heat also, if 
sufficiently intense and if absorbed with sufficient avidity, could produce all the effects 
of ordinary thermometric heat. The dark rays, for example, which can make platinum 
white-hot, could also, if absorbed, produce the chemical effects of white-hot platinum. 
They could decompose water, as now in a moment they can boil water. But the decom- 
position in this case would be effected by the virtual conversion of the radiant heat into 
thermometric heat. There would be nothing in the act characteristic of radiation, or 
demanding it as an essential element in the decomposition. 
The dark calorific rays are powerfully absorbed by various bodies, but, as a general 
rule, they do not appear competent to set up that particular motion among the consti- 
tuents of a molecule which breaks the tie of chemical affinity. All the rays of the 
spectrum exercise no doubt chemical powers. We should have scant vegetation upon 
the earth’s surface if the red and ultra-red rays of the sun were abolished. But the 
chemical actions in which the radiant form conies into play, are mainly produced by 
the least energetic rays of the spectrum. The photographer has his heat focus in ad- 
vance of the chemical focus; which latter, though potent for his special purposes, 
possesses almost infinitely less mechanical energy than its neighbour. Some special 
relation must, therefore, as a general rule, subsist between chemical molecules and the 
more refrangible rays; we arrive at the conclusion that chemical decomposition by 
rays, to keep to the ordinary term, is less a matter of amplitude on the part of the 
vibrating ether particles than of time of vibration. 
The decomposition of a molecule must result from the internal strain of its parts ; to 
them, therefore, and not to the molecule as a whole, the vibrations which produce 
chemical change must be imparted. The question remains an outstanding one in mole- 
cular physics, why it is that the longer and more powerful ether waves are generally 
incompetent to set up the motion which results in decomposition. The influence of 
* It may be a millionfold; for we do not yet know bow small tbe absorption of tbe absolutely pure mixture 
really is. 
