Radiant Heat, and its Conversion thereby into Sound. 455 



added the subsequent testimony of Professor Wild, now of 

 the University of St. Petersburg, who went carefully over 

 the same ground. " In all my experiments," he says, " con- 

 ducted according to TyndalFs 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 " *. 



In an extremely able paper, a translation of which is pub- 

 lished 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 th.Q method of Magnus. iC Although/' 

 he continues, " 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 TyndalFs 

 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 TyndalFs 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 determinations of the ra- 

 diation 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 "f. He then describes his arrangement : — " The 

 gases and vapours were passed through a brass tube of 

 15 millimetres internal diameter, which was placed horizon- 

 tally and heated by gas-flames. One end of the tube was 

 bent upwards, so that the heated air ascended vertically, 

 while at a distance of 400 millimetres 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 



* Phil. Mag. 4th ser. vol. xxxii. p. 252. 



t This is an inadvertence. Exhaustive experiments on the radiation 

 of gases and vapours had been made and published many years previously 



