242 H. Wild on the Absorption of 



scription of my experiments will show how far I have been suc- 

 cessful in deciding this question. 



It must first be remarked that the results which the two inves- 

 tigators have obtained, each by his own method, do not differ, on 

 closer consideration, so much as at first they may appear to do. 

 Professor Magnus concludes, for instance, from his experiments 

 with dry air and with air saturated at 16° C. with aqueous va- 

 pour, that the aqueous vapour existing in the air at this tempera- 

 ture exercises no perceptible influence on the absorption*. The 

 stratum of air which was here interposed between the source of heat 

 at 100° and the thermopile was 1 foot in thickness f. The deflec- 

 tions of the galvanometer-needle amounted respectively to 12°'5 

 and 12°'6; the difference therefore falls below the mean error of 

 observation, o, 2. On the other hand, Professor Tyndall con- 

 cludes from his measurements J that a stratum of air 4 feet in 

 length, and saturated with aqueous vapour, absorbs, in round 

 numbers, 10 per cent, of the whole radiation ; and moreover 

 this number holds good for the experiments where the tube was 

 not closed by plates of rock-salt. Of every 100 incident rays, 

 90 passed through; or, to express it more correctly, if we denote 

 by 1 the heating effect of the incident rays, that of the emergent 

 ones was 0*90. Let us assume, as usual, that equally thick strata 

 absorb equally, which is certainly admissible in the case of 

 feebly absorbing moist air ; then, according to this, out of every 

 100 incident rays 97*5 would pass through a stratum of moist 

 air only 1 foot in thickness • in other words, the heating effect of 

 the emergent rays would be to that of the incident rays as 0'975 

 to 1. This decrease of the heating effect would, in the experi- 

 ments of Professor Magnus, have corresponded to a diminution 

 of the deflection amounting to o, 4 ; so that the difference be- 

 tween the statements of the two investigators reduces itself in 

 reality to a magnitude which does not exceed double the error of 

 observation in one of the investigations. 



An actual contradiction, however, exists between the results 

 which Professor Magnus obtained, on experimenting according to 

 the method of Professor Tyndall § with a tube 0*66 of a metre 

 long and open at both ends, and the statements of Professor 

 Tyndall himself. 



The deflections of the galvanometer-needle, which Professor 

 Magnus observed on alternately introducing dry and moist air, 



* Phil. Mag. August 1861, p. 106. 



t I merely consider here the experiments made by Professor Magnus 

 with the source of heat at 1 00°, and omit those made with the gas-lamp, 

 since the latter do not admit of being compared with Professor TyndalFs 

 experiments. 



X Pogg. Ann. vol. cxviii. p. 575. [Phil. Mag. July 1863, p. 21.] 



§ Phil. Mag. S. 4. vol. xxvi. p. 24. 



