\\V1 



sally received constitution of the molecules of oxygen and ozone, the 

 molecule of ordinary oxygen containing 2 atoms, whilst the molecule 

 of ozone contains only three. 



Whilst Tyndall was pursuing these researches in London, Magnus 

 was engaged in conducting a similar investigation in Berlin. The 

 general agreement between the results of these two able experi- 

 menters was, as might be expected, very close. In one important 

 respect, however, there was a striking divergence. This was in regard 

 to the action of aqueous vapour upon radiant heat, Magnus having 

 found that aqueous vapour had little or no action, whilst Tyndall 

 found it to be a very powerful absorbent of the heat rays of low re- 

 frangibility. A long controversy ensued, each experimenter appeared 

 to have full confidence in his own results, and the opinions of others 

 were consequently far from unanimous until a paper by Tyndall, 

 published in the ' Proceedings of the Royal Society ' in 1881, and 

 entitled "Action of an Intermittent Beam of Radiant Heat upon 

 Gaseous Matter," finally decided the point, and proved in a startling 

 manner that Tyndall was right. In this paper he describes his repe- 

 tition of the ingenious experiments of Mr. Graham Bell, wherein 

 musical sounds were obtained through the action of an intermittent 

 beam of light on solid bodies. Entertaining the opinion that these 

 singular sounds were caused by changes of temperature producing cor- 

 responding changes of shape and volume in the bodies impinged upon 

 by the beam, Tyndall argued that if this be the case, and if gases and 

 vapours really absorb radiant heat, they ought to produce sounds more 

 intense than those obtainable from solids ; and it seemed to him plain, 

 moreover, that by this new method many of his previous results 

 might be brought to an independent test. Highly diathermanoua 

 bodies, he reasoned, would produce faint sounds, while highly 

 athermanous bodies would produce loud sounds; the strength of 

 the sound being, in a sense, the measure of the absorption. The 

 source of the intermittent beam was a Siemens lamp connected with 

 a dynamo machine in the front of which was placed a rotating disk. 

 The result shall be related in Tyndall's own words. 



" Sulphuric ether, formic ether, and acetic ether, being placed in 

 bulbous flasks, their vapours were soon diffused in the air above the 

 liquid. On placing these flasks, whose bottoms only were covered by 

 the liquid, behind the rotating disk so that the intermittent beam 

 passed through the vapour, loud musical tones were in each case ob- 

 tained. These are known to be the most highly absorbent vapours 

 which my experiments revealed. Chloroform and bisulphide of 

 carbon, on the other hand, are known to be least absorbent, the latter 

 standing near the head of diathermanous vapours. The sounds ex- 

 tracted from these two substances were usually weak and sometimes 

 barely audible, being more feeble with the bisulphide than with the 



