PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
of the thermometer, when A B was exhausted/” was compared with its heating, when 
A B was filled with various gases, and in every case but one the heating through the gas 
was found less than the heating through the vacuum. The exception was hydrogen, 
which carried more heat to the thermometer than was transmitted by the vacuum. 
The conclusion drawn by its author from this experiment was that hydrogen conducted 
heat like a metal. 
One rema,rk only in this Note has any reference to the diathermancy of gases, but it 
is a significant one. Magnus had no doubt as to the power of every one of his gases 
to conduct heat. There could, he supposed, be only a difference of degree between 
them and hydrogen. Whence, then, the lowering of the thermometer ? He answers 
thus :—“From this it is not to be inferred that the gases do not conduct heat, but 
merely that in their case conduction is so feeble as to be neutralised by adiathermancy.” 
These are the only words in the note which have any reference to radiation. 
In his next inquiry, Magnus dealt directly with the subject of diathermancy, a 
Preliminary Note of the investigation being published in the Monatsbericht for 
February 7th, 1861. This note, like its predecessor, consisted of general and descriptive 
statements, no actual measurements being given. The completed memoir was first 
published in Poggendoiiff’s Annalen for April, 1861. For the purposes of this new 
inquiry the apparatus used in the experiments on gaseous conduction was modified as 
shown in fig. 2. To the recipient A B a second one G F was attached, both being 
connected by the tubulure shown in the figure. The recipient G F rested upon the 
plate of an air-pump, on which also stood the thermopile p, with one of its faces turned 
towards the source. From the pile, through the air-pump plate, wires passed to the 
galvanometer. With this apparatus, the absorption by atmospheric air and by oxygen 
was found to be 11T2, and by hydrogen 14T per cent, of the total radiation. The 
alleged conductivity of hydrogen did not therefore manifest itself in these experiments. 
Let us analyse these results. In the first experiments, the distance of the thermo¬ 
meter from the source of heat was 35 millimeters. The action on the thermometer 
through a vacuum being represented by 100, the action through air and through 
oxygen of this depth was found to be 82. The loss of 18 per cent, in air and in 
oxygen was alleged to be due to the adiathermancy of these media, to which per¬ 
centage, if we wish to ascertain the total absorption by air, we should have to add 
such heat as reached the thermometer by conduction. 
Turning now to the modified apparatus, which is evidently drawn to scale, the gas 
here traversed by the radiant heat was about 275 millimeters in depth, while the 
stratum traversed in the first experiments was, as stated, only 35 millimeters. Yet 
in these first experiments an absorption of 18 per cent., while in the later ones an 
absorption of only 11'21 per cent, is assigned to air. In other words, when the depth 
of the aerial stratum was augmented more than seven-fold, the absorption, instead of 
# This vacuum temperature, at least iu so far as it exceeded that of the sides of the recipient, was 
obviously derived from the screen. 
