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PROFESSOR TYRDALL OR THE ACTION OF FREE MOLECULES OR 
Wild, now of the University of St. Petersburg, who went carefully over the same 
ground. “ In all my experiments,” he says, “ conducted according to Tyndall’s 
method, which included more than a hundred distinct observations, I have never 
obtained deflections of the galvanometer needle in contradiction to the statements of 
Professor Tyndall.”' 5 ' 
In an extremely able paper, a translation of which is published in the Philosophical 
Magazine for October, 1866, the Petersburg philosopher compares the methods pursued 
by Magnus and myself respectively. Insufficient sensitiveness and the disturbance 
due to convection currents, caused him, he says, to abandon the method of Magnus. 
Although, he continues, c: this method of investigating absorption may, in the hands 
of so experienced and expert an experimenter as Professor Magnus, be an appropriate 
one for determining absolute values with great certainty, I feel bound, from my own 
experience, to give a decided preference to Tyndall’s method, not only on account of 
the greater facility with which it furnishes qualitative [quantitative] results, but also 
in consequence of its greater delicacy. It is principally in consequence of this greater 
delicacy that, notwithstanding the negative results furnished by Magnus’s method, I 
maintain that the greater absorptive power of moist air, as compared with dry, has 
been fully established by the experiments made according to Tyndall’s method; and 
I am of opinion that meteorologists may without hesitation accept this new fact in 
their endeavours to explain phenomena which hitherto have remained more or less 
enigmatical.” 
In 1866 Magnus varied his method of experiment, seeking to solve the question of 
absorption by observations on radiation. “ I have,” he says, “ made a few determina¬ 
tions of the radiation of dry and moist air, and of some other gases and vapours. Up 
to the present time,” he continues, “the capacity of these bodies to transmit heat has 
alone been determined.”! He then describes his arrangement:—“The gases and 
vapours were passed through a brass tube of 15 millimeters internal diameter, which 
was placed horizontally and heated by gas flames. One end of the tube was bent up¬ 
wards so that the heated air ascended vertically, while at a distance of 400 millimeters 
from the vertical current, was placed the thermopile.” When dry air was sent through 
this tube, the deflection produced was three divisions of a scale; when air which 
had passed through water at a temperature of 15° C. was sent through the tube, the 
deflection rose to five divisions; when the water was warmed to 60° or 80° Fahr., 
the deflection was 20 divisions; and when the water boiled, the deflection was 
100 divisions. In this last experiment, however, a mist appeared, so that, as urged at 
the time, the radiation could not be said to have been purely from vapour. In the 
other cases no mist was visible, but it was nevertheless concluded that the 20-division 
deflection was due to the formation of mist at the boundary of the ascending current. 
* Phil. Mag., 4th ser., vol. 32, p. 252. 
f This is an inadvertence. Exhaustive experiments on the radiation of gases and vapours had been 
made and published many years previously. 
