310 The Rey. Dr. Rosinson on the Effect of Heat, &c. 
ordinary cases it requires the temperature to be raised before it acts; yet even 
then is it not probable that, if not light, some other actinic power (like that which 
accompanies light in the spectrum, and is revealed to us by its chemical effects in the 
processes of photography ) is evolved by the heat, and, though invisible, determines, 
in conjunction with the affinity, that atomic change which transforms the three 
volumes of oxygen and hydrogen into two of steam? This conjecture seems to 
me to derive some support from the singular fact, that when a platinum wire is 
traversed by a current gradually increased till it produces ignition, the first gleam 
that appears is not red,* but of a colour which, when I first saw it, I compared to 
the “lavender ray” discovered by Sir John Herschel beyond the violet, though 
I was surprised at seeing the tint of that most refrangible ray preceding the ray 
which is least so. It is quite conspicuous at about 865°; and as the mode in which 
it makes its appearance presents nothing abrupt or discontinuous, it seems likely 
that it is merely a transition from invisible rays, excited at a lower temperature, to 
ordinary light. These are certainly mere surmises, nor would I occupy the time 
of the Academy with them, but that I hope some of them may prompt experi- 
mental inquiry in a direction where, at present, our ignorance is almost total. 
With the same view I will add to them a suggestion, which I think deserves the con- 
sideration of some of those distinguished mathematicians who adorn our Society. 
The undulatory hypothesis is as decidedly connected with radiant heat as 
with light. Can it be also applied to heat when traversing bodies by conduction ; 
and can it be brought to bear on that class of phenomena which exhibit it as 
becoming latent in various changes of molecular condition, and again reappearing 
with the definite numerical precision of a chemical equivalent? Such an attempt, 
if successful, would probably unlock a whole treasure of hidden knowledge; and 
it would scarcely be less precious were its failure to show that in these three forms 
of heat we have been confounding modes of energy which are essentially distinct. 
ArMAGH OBSERVATORY, 
June 11, 1847. 
* This fact has recently been published by Dr. Draper in a paper containing much remarkable 
matter, and, I may add, curiously coincident with some of my investigations, with which, of 
course, it was impossible for him to be acquainted. I observed it in September, 1844, while making 
experiments on the application of wires faintly ignited as the lines of a micrometer, which I pro- 
posed to use with Lord Rosse’s telescopes for drawing nebule. This mode of illumination would 
be perfect were it not that the heated air from the wires produces a little indistinctness. 
A 
