310 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
of my experiments. These, as I have so frequently stated, were conducted with plates 
of salt as dry as polished glass or rock crystal. Thus, while conceding the fact that 
rocksalt is hygroscopic, I demur to its application. 
A similar remark applies to the last solution offered by Magnus of the differences 
between us. In 1867, he showed that vapours were condensed by surface attraction 
to a greater extent than had previously been supposed. Blowing vapour-laden air 
into a metal tube, he found that heat was generated. He inferred, and rightly 
inferred, that this heat was produced by the condensation which occurred on the 
interior surface. This condensation he found to depend on the condition of the 
surface, being greater when it was tarnished or coated than when it was polished. 
He saturated air with moisture at a temperature of 16° C., and then raised both it 
and his pile to a temperature of 38°. When such air was blown against the dry face 
of the pile heat was generated. Condensation therefore occurred upon a surface 
22° C. higher than the dew point of the vapour. Against this I have nothing to urge. 
But the fact by no means justifies the inference drawn from it, which was, that the 
vapours in my experimental tube were converted by “ vapour-hesion ” into liquid 
layers of high opacity to radiant heat. These layers, acting upon the calorific rays 
impinging on the interior surface of my tube, produced the absorption which I had 
erroneously ascribed to the vapours. More than this, the liquid layers were supposed 
to be broken up into discontinuous patches which not only absorbed the heat but 
scattered it. “ Vapour-hesion/’ it may be added, was found to vary with the liquid 
which produced the vapour, being particularly strong in the case of alcohol. 
Magnus brought this generalization to the test of experiment, but failed to verify 
it. He urged humid air against a dry mirror, from which radiant heat was reflected; 
but unless he wetted the mirror visibly no effect was produced on the reflected beam. 
Still he held that reflection, oft repeated, rendered sensible an action which eluded a 
single reflection. My position here is clear. I do not doubt surface attraction, or 
deny the existence of impalpable films. No experiment was ever made on the reflec¬ 
tion of light or radiant heat in which such films did not intervene, but they had as 
little effect upon my results as they had upon those of De la Provostaye and 
Desains,* and of other refined experimenters. As early as 1859, I was made aware 
of the danger which might arise from condensation. Warned by the action of chlorine 
on my brass experimental tube, I coated it inside with lampblack, and re-tested with 
it all my vapours. The result removed from my mind the suspicion that surface 
condensation had anything to do with the observed absorption. Many similar 
experiments with blackened tubes were subsequently made by me for my own safety 
and instruction. There was no substantial difference between the results obtained 
* Considering the energy of water as a radiator, exceeding according to Leslie that of lampblack 
itself, the film of this liquid which must have covered the plates of silver in the experiments of the two 
French philosophers ought, if Magnus he correct, to have sensibly raised the emission. Calling the 
emission from lampblack 100, that from polished silver plus the film was only 2T, 
