344 
PROFESS OE TYNDALL ON THE ACTION OF RAYS 
diminish very materially the power of the beam. A passage through 1 foot of the nitrite 
of amyl would have been far more destructive. 
As these actions are representative and, I believe, important, I will here sum up 
some recent confirmatory experiments executed with the two substances now under 
consideration. 
1°. The vaporous nitrite of amyl absorbs with such avidity the rays competent to 
decompose it, that a very small depth of the vapour quenches the efficient rays of a 
powerful beam of solar or electric light. 
2°. The vaporous iodide of allyl, on the contrary, permits a beam to traverse it for 
long distances without very materially diminishing the chemical power of the beam. 
3°. The liquid nitrite of amyl, in a stratum one quarter of an inch thick, quenches all 
the rays which could act chemically upon its vapour. 
4°. The liquid iodide of allyl, on the contrary, in a stratum of four times the thick- 
ness j ust mentioned, does not materially diminish the power of the beam to act upon its 
vapour. 
5°. A very marked difference exists between the deportment of the nitrite of amyl 
alone, and its deportment when mixed with hydrochloric acid. The chemical penetrability 
of the mixture is far greater than that of the pure vapour. The actinic cloud, which 
with the vapour alone is confined to the anterior portion of the experimental tube, 
extends in the case of the mixture through the entire tube. 
6°. A beam, moreover, which has been transmitted through a quarter of an inch of 
the liquid nitrite is also competent to act chemically upon the mixture, and to produce 
in it dense actinic clouds. 
The action in this last case, though not stopped by the liquid nitrite, is retarded. 
Employing first the liquid screen, it was interesting to observe the sudden development of 
a fine-grained luminous cloud, and its violent tumbling about by the decomposing beam 
the moment the liquid was withdrawn. The action of a solution of the yellow chromate 
of potash is substantially the same as that of the liquid nitrite. By the successive intro- 
duction and removal of a cell containing either substance, successive flashes of actinic 
energy may be produced a dozen times and more in the same vapour. 
The molecular relationship of a liquid and its vapour receives new illustration from 
these experiments. Whatever alters the action of the one appears to change in a 
proportionate degree the action of the other. 
§ VIII. 
Carbonic acid is decomposed by the solar beams in the leaves of plants ; but here it 
is in nresence of a substance, chlorophyll, ready, as it were, to take advantage of the 
loosening of the atoms by the solar rays. The present investigation has furnished nume- 
rous cases of a similar mode of action. All the vapours examined may be more or less 
powerfully affected in their actinic relations by the presence of a second body with 
which they can interact. The presence, for example, of nitric acid, or of hydrochloric 
