Paper of Professor Magnus. 119 



obvious reflections suggested by his own description of his ex- 

 periments. 



Professor Magnus now infers the absence of absorption from 

 the absence of radiation. He employs as source of heat a stream 

 of air which is first urged through water at the ordinary tempe- 

 rature, and afterwards caused to pass through a hot brass tube 

 15 millims. in diameter. On its emergence from the tube it 

 radiates against a thermo-electric pile placed at a distance of 400 

 millimetres. When dry air was urged through the tube, the de- 

 flection was exceedingly small ; when air moistened as above was 

 employed, the deflection was scarcely augmented. 



Now, in the first place, the amount of vapour taken up by air 

 in its passage through cold water is so small, and the stream of 

 such air employed by Professor Magnus is so thin, that the heat 

 radiated from the vapour must be excessively minute. Suppo- 

 sing the vapour compressed to the density of ordinary atmo- 

 spheric air, the average thickness of the radiating layer would 

 probably be less than -3^ of an inch. Even assuming the 

 rays from this source to reach the pile without impediment, its 

 action would be inconsiderable, if not insensible. 



But the rays were not permitted to reach the pile without 

 impediment. I assume that Professor Magnus did not deem it 

 necessary to dry the air intervening between his source and his 

 pile ; otherwise he would have mentioned a precaution of such 

 importance. Here, then, we have the vapour of a column of air 

 15 millims. thick radiating through the vapour of a layer of 

 air 400 millims. thick, that is to say, through twenty- seven 

 times its own thickness of a substance intensely opake to the 

 radiation. Considering, then, the feebleness of its origin and 

 the difficulties in its way, it is not surprising that the radiation 

 from the source chosen by Professor Magnus failed to produce 

 any very sensible impression upon his galvanometer*. 



It must be borne in mind that, to obtain copious radiation 

 from a substance so attenuated as aqueous vapour, a considerable 

 length of it must be employed. An example will illustrate this. 

 When enclosed in a tube 3 feet long, the radiation of sulphuric 

 ether vapour, at 0*5 of an inch of pressure, exceeds that of olefiant 

 gas at the pressure of the atmosphere. In a tube 3 inches long, 

 on the contrary, the radiation from the gas is more than treble 

 that from the vapour j\ That carbonic acid gas excels aqueous 



* I have no means of judging the humidity of the radiating air as com- 

 pared with that of the air through which it radiated. If the water em- 

 ployed to saturate the air were very cold., the latter might be the greater of 

 the two. 



t Heat as a Mode of Motion, 2nd edit. p. 381. 



