96 J. W. Draper—Distribution of 
heat should be instantaneously disengaged by the preliminary 
actinization, it is difficult to conceive how it can so instantane- 
ously disappear. 
hen the radiation is withdrawn, and the hydrochloric acid 
absorbed, there is no after-combining. The action is perfectly 
definite. For a given amount of chemical action, an equiva- 
lent quantity of the radiation is absorbed. 
The instances I have cited in this discussion of the mode of 
action of radiations are, one of decomposition, in the case of the 
silver iodide, and one of combination, in the case of hydrochloric 
acid. I might have introduced another, the dissociation of 
ferric oxalate, which I have closely studied, but it would have 
made the memoir of undue length. From the facts herein con- 
sidered the following deductions may be drawn. 
When a radiation impinges on a material substance it im- 
parts to that substance more or less of its vis viva, and therefore 
undergoes a change itself. The substance also is disturbed. 
Its physical and chemical properties determine the resulting 
phenomena. 
(1st.) Ifthe substance be black and undecomposable, the radia- 
tion establishes vibrations among the molecules it encounters. 
We interpret these vibrations as radiant heat. The molecules 
of the medium do not lose the wis viva they have acquired at 
once, since they are of greater density than the ether. Each 
becomes a center of agitation, and heat-radiation and conduc- 
tion in all directions are the result. The undulations thus set 
are commonly of longer waves, and as the movements grad- 
ually decline the shorter waves of these are the first to be extin- 
guished, the longer ones the last. This, therefore, is in accora- 
ance with what I found to be the case in the gradual warming 
of a solid body, in which the long waves pertain to a low tem- 
perature, the short ones arising as the temperature ascends 
hil. Mag., May, 1847). : 
In some cases, however, instead of the disturbing undulation 
giving rise to longer waves, it produces shorter ones, as is shown 
when a platinum wire is put into a hydrogen flame, or by Tyn- 
dall’s experiments, in which invisible undulations below the red 
ive rise to the ignition of platinum. : 
(2d.) If the substance be colored and undecomposable, it will 
extinguish rays complementary to its own tint. The tempera- 
ture will rise correspondingly. 
(3d.) If the substance be decomposable, those portions of the 
radiation presented to it which are of a complementary tint 
wil] be extinguished. The force thus disappearing will not 
expended in establishing vibrations in the arresting particles, 
but in breaking down the union of those which have arrested 
