88 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
of a column of air fifteen times the length of that employed by Professor Magxus was 
absolutely insensible. 
In the account of the experiments already published, where my source of heat was 
also 100° C., I have set down the absorption of air, oxygen, and hydrogen at about 0‘33 
per cent. ; which is for air and oxygen thirty times, and for hydrogen over forty times 
less than that found by Professor Magnus. 
In fixing the above figure for the absorption of these gases, I protected myself by 
assigning what I knew to be the superior limit of the effect, but I was morally certain 
at the time, that as soon as I could combine sufficient power and delicacy I should make 
the effect less. This I have done in my present inquiry, and find the absorption of the 
above gases to be under OT per cent., which in the case of oxygen is less than y^th, 
and in the case of hydrogen less than of effect obtained by Professor Magnus 
with a tube less than half the length of mine. Making every allowance for the differ- 
ence between our two sources of heat, the discrepancy between us. is still enormous. In 
fact my conclusion is that these gases are practical vacua to radiant heat, and that the 
mixture of oxygen and nitrogen which constitutes the body of our atmosphere is the 
same. 
While, however, in the case of the elementary gases the discrepancy between Professor 
Magnus and myself consists in a defect on my part, or an excess on his, with the 
powerful gases I obtained a considerably stronger action than he does. Thus with 
olefiant gas his absorption amounts to less than 54 per cent., whereas in m ine it amounts 
to more than 72. This last result is what might only be expected, inasmuch as the 
length of gas traversed by the radiant heat was in the one case a little under 15 inches, 
and in the other 33. 
Professor Magnus has further published an account of experiments in which a powerful 
gas-flame surrounded by a glass cylinder furnished the source of heat ; the latter being 
augmented by a parabolic mirror of polished metal, placed behind the lamp. In this 
case the gases were enclosed in a glass tube 1 metre long and 35 millims. in diameter, 
the two ends of which were stopped with plates of glass 4 millimetres thick. 
Two series of experiments were executed with this tube, in one of which the interior 
surface was covered with black paper, while in the other the glass was uncovered within. 
The former method is that pursued by Dr. Feanz, and the result obtained by Professor 
Magnus in the case of atmospheric air and oxygen closely agrees with that obtained for 
the same gases by Dr. Feanz. Professor Magnus makes the absorption in the case of 
the blackened tube about 2y, and Dr. Feanz about 3 per cent, for air and oxygen. 
In the case of the unblackened tube, however, the absorption was found to be much 
more considerable. Here the absorption by air and oxygen amounted to 14’ 75 per cent., 
and with hydrogen it reached 16 ‘23. This great difference between the unblackened 
and the blackened tube is ascribed by Professor Magnus to a change of quality which 
the heat undergoes by its reflexion from the interior glass surface. 
One of my motives in introducing a glass tube into the present inquiry was, that I 
