322 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
accomplish, I hope, in testing the action of humid air, that the use of the silvered tube, 
closed with polished plates of rocksalt, on which no trace of. visible moisture is 
deposited, will be conceded to me. This tube involves the use of a source of heat of 
small dimensions. The lime light, though fulfilling this condition, is not suitable, 
because of the high refrangibility of its heat. The incandescent platinum spiral would 
be better; still, the radiation from this source is but feebly absorbed by the aqueous 
vapour taken up by air at ordinary temperatures. The oxyhydrogen flame fulfils the 
required conditions best.* It has the advantage of high temperature and low re¬ 
frangibility, while the fact that its heat, as coming from aqueous vapour, is absorbed 
with special energy, by aqueous vapour, is also a strong recommendation. In the 
Bakerian Lecture for 1864, I have illustrated this point. It was then shown that 
when a platinum spiral, rendered incandescent by a voltaic current, had 5*8 per cent, 
of its heat absorbed by un-dried air, a hydrogen flame had from 17 to 20 per cent, of 
its heat absorbed. The mere plunging of a platinum spiral into the flame caused the 
absorption to fall from 17 per cent, to 8 "6 per cent. Hence my reason for choosing a 
hydrogen flame in the present instance. 
Dry air and humid air being caused to occupy the experimental tube in succession, 
both of them were compared with the radiation through the tube when very perfectly 
exhausted by a Bianchi’s pump. The following results were obtained. 
Vacuum. 
Deflection. 
o-o° 
Absorption per 100. 
0-0 
Dry air. 
o 
O 
o 
0-0 
Humid air. 
00 
o 
o 
11*7 
Vacuum. 
o 
O 
O 
0*0 
Dry air. 
o 
O 
O 
o-o 
Humid air. 
7*6° 
11*2 
The deflection through a vacuum, otherwise the total heat, was 47° which, according 
to my calibration table, is equivalent to 68 units. 
A fresh supply of hydrogen was here introduced into the gas holder. The total 
heat being 46° or 65 units, the following results were obtained. 
Deflection. 
Absorption per 100. 
Dry air. 
o 
p 
o 
0*0 
Humid air. 
.... 6-2° 
9’5 
Dry air. 
.... 0*0° 
o-o 
Humid air. 
.... 6-7° 
10'3 
Mean of the four determinations with humid air 10’7. 
* I was careful to assure myself that, unless it amounted to the visible wetting of the plates of salt, 
there was no sensible stoppage of the rays from the oxyhydrogen flame. This quite agrees with the 
result obtained by Magnus himself, in the experiment with the concave mirror already referred to. 
Unless he visibly wetted the mirror he failed to impair the energy of the reflected beam. 
