304 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
narrow tubulure. Through it the currents could not pass, but they nevertheless 
existed in the recipient A B, lowering the temperature of the source without heating 
the thennoscopic instrument. 
The experimental resources of Magnus were great, and he here applied them; but 
the defects of his method were radical and irremovable. These defects reached their 
culmination in the subsequent researches of Professor Buff,* who by pursuing sub¬ 
stantially the same method, arrived at the result that a stratum of air inches thick 
absorbed 60 per cent, of the radiation from a source of 100° C.t Buff also found 
olefiant gas to be more diathermanous than air, whereas at atmospheric pressures, 
it is many hundred, and at a pressure of ^th of an atmosphere, many thousand times, 
more opaque to heat. 
This is the point at which aqueous vapour enters into the experiments of 
Magnus. When dry and humid air were compared together in his apparatus, no 
difference between them was observed. But, apart from all disturbance, it would 
require an instrumental arrangement far more delicate and powerful than that here 
employed, to bring into view the action of a stratum of mixed air and aqueous vapour 
11 inches deep, and having a temperature of only 15° C. Disturbances, however, were 
not absent. In the first place, the convection currents which enabled dry air to reduce 
the radiation by 11T2 per cent., were more than sufficient to mask the action of the 
vapour. Secondly, dry and humid air were brought in succession into direct contact 
with the face of the thermopile. The pile was therefore affected by any difference of 
temperature between it and the air, and it could scarcely be supposed that these 
temperatures were always alike. It was also affected by the condensation and 
evaporation which occurred when humid air and dry air were brought successively 
into contact with its lampblack-coated face. To “ vapour-hesion ” Magnus subse¬ 
quently ascribed very large effects. Here we have the conditions specially suited to 
the development of the action, and yet no reference is made to it. Either then the 
disturbance was overlooked, or the apparatus was not sufficiently delicate to reveal it. 
To these two sources of disturbance—the lowering of the temperature of the source 
by convection, and the warming and chilling of the pile by contact, condensation, and 
evaporation—is to be added another, due to the warming which must have occurred 
when the dynamically heated air came into direct contact with the thermopile—an 
action which, in my apparatus, proved sufficient to whirl the needle of the galvano¬ 
meter more than once through an entire circle. 
Magnus next experimented with glass tubes 1 meter long and closed at the ends 
with plates of glass. His source of heat was a strong gas flame aided, as in the 
experiments of Dr. Franz, by a parabolic mirror. Two tubes were employed, the one 
blackened within and the other unblackened. With the blackened tube an absorption 
* Phil. Mag., 5th ser., vol. 4, p. 401. For my l’eply see Proc. Roy. Soc., vol, 30, p. 10. 
f Ibid., 1877, vol. 4, p. 424. 
