252 H. Wild on the Absorption of 



vapour of ether, to the Scientific Society at their Meeting on the 

 10th of June. From the deflections of the galvanometer-needle 

 the members were able to verify the fact of the greater absorptive 

 power of moist air compared with that of dry air. 



In conclusion, I may say that in all my experiments conducted 

 according to TyndalPs 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, that, further, my measurements give approxi- 

 mately the same ratio of the absorption by moist air to that by 

 coal-gas, and that, lastly, I consider that certain objections 

 which might have been raised against the conclusiveness of Pro- 

 fessor Tyndall's experiments have been removed by means of ap- 

 propriate changes in his method of experimenting. 



This complete verification of Tyndall's results rendered it more 

 desirable to investigate the absorption by dry and by moist air 

 by the method of Professor Magnus. 



2. Experiments by Professor Magnus's Method. 



The characteristic features of Professor Magnus's method are 

 that the thermopile is altogether within the tube destined to re- 

 ceive gases, and that this tube is closed at its ends by the sources 

 of heat themselves. By this means we at once secure a contact 

 between the gas and both sides of the thermopile, as well as be- 

 tween the gas and both the sources of heat; and, moreover, 

 quite definitively defined strata of gas can be interposed between 

 the sources of heat and the thermopile without using plates of 

 rock-salt or other closing plates, which might disturb by their 

 own absorption. 



In order to combine the advantages of this method with those 

 of TyndalPs, 1 had in the first place a tin tube prepared, about 

 A feet long and 9 centims. wide ; each of its ends was closed 

 by one of the sides of a cube-shaped vessel of zinc, in which 

 water was kept boiling by means of the introduction of vapour. 

 At a distance of a foot from one end of the tube the above-de- 

 scribed thermopile, deprived of its conical reflectors and its foot, 

 lay freely in the horizontal axis of the tube, held by the pole 

 wires. Between the pile and the nearest end of the tube were 

 a diaphragm and a disk, of which the latter could be turned from 

 the outside, in order, when requisite, to weaken the radiation of 

 the nearest source of heat ; on the side of the other source of 

 heat were four similar diaphragms at about equal distances from 

 one another. These diaphragms, as well as a vessel surrounding 

 the tube up to near the sources of heat, and containing cold water, 

 were intended to prevent heat being imparted to the thermopile 

 by conduction or convection. In reality, however, this was so 



