298 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
compare together the various gases at very small pressures. When the pressure was 
that of 1 inch of mercury the diathermic range was greatly increased, the absorption 
by olefiant gas being then at least 6000 times the absorption by atmospheric air. 
With the changed apparatus the action of the aqueous vapour of our atmosphere 
was again taken up, and proved to be, not 13 times, as I had at first supposed, but, on 
fairly humid days, at least 60 times that of the air in which it was diffused. When, 
moreover, dried air was caused to pass over moistened glass and then carried into the 
experimental tube, the absorption was still greater. 
A power has been claimed for mist or haze which has been denied to aqueous vapour; 
but in these experiments concentrated luminous beams, which would have infallibly 
brought into view the least trace of suspended matter, revealed no mist or dimness ol 
any kind. It is, moreover, demonstrable that an amount of turbidity rendered strikingly 
evident by a luminous beam, exerts only a fractional part of the action of the pure 
aqueous vapour. When well dried air was led, not through water or over wet glass, 
but over bibulous paper, taken apparently dry from the drawers of the laboratory, the 
amount of vapour carried forward from the pores of the paper produced 72 times the 
absorption of the air which carried it. After five repetitions of the experiment, 
wherein the same air was carried over the same paper, a quantity of vapour was still 
sent forward capable of exerting 47 times the absorption of the air in which it was 
diffused. 
Here the possible action of odours upon radiant heat naturally suggested itself. 
Many perfumes were accordingly subjected to examination, the odorous substance 
being in each case carried into the experimental tube by a current of dry air. 
Thus tested pachouli exerted 30 times, cassia 109 times, while aniseed exerted 372 
times the absorption exerted by the air in which it was diffused. 
A novel method of exhibiting the absorption and radiation of gaseous bodies, the 
germ of which had been previously discovered,* was illustrated and developed by the 
new apparatus. Suppose the experimental tube exhausted and the needle, under the 
joint action of the two sources, to be at 0°. On admitting a strong vapour the usual 
deflection would occur. Suppose it to be 50 galvanometric degrees. Let dry air be 
now introduced until the experimental tube is filled. Although fresh matter is thus 
thrown athwart the rays of heat, the needle behaves as if the matter within the 
experimental tube had wholly disappeared. It sinks to zero, and not only so, but 
passes say to 50° on the other side. 
After the first moments of perplexity succeeding the observation of this effect, its 
cause became clear. On entering the experimental tube the air, having its vis viva 
destroyed, was heated dynamically. Incompetent to radiate itself, it imparted its 
warmth to the vapour, and this powerful radiator poured the heat thus received 
against the pile. This heat sufficed not only to neutralise the deflection of 50° due to 
* Philosophical Transactions, Vol. 151, p. 32. 
