1879.] Buff's Experiments on the Diathermancy of Air. IS 



quotients obtained with different gases would express their relative 

 actions. It is, I suppose, on considerations such as these that Pro- 

 fessor Buff has based his column of " comparable values," where the 

 galvanometric value is multiplied by 100 and divided by the thermo- 

 metric value. 



But the actual experiments of Professor Buff are by no means so 

 simple as the ideal ones here described. He brings, first of all, his 

 needle to zero by the introduction of water of the proper temperature 

 into his brass vessel. Removing a portion of this he pours in hot 

 water, and has to wait for 22 minutes when air is in his cylinder, and 

 for 15 minutes when the cylinder is exhausted, before his needle attains 

 its maximum deflection. Here it remains stationary " for 30 minutes 

 and more." Thus while the temperature of the polished brass bottom 

 is steadily sinking from 56° to 47 0, 7, the deflection is gradually 

 rising, obviously not through the action of what was intended to be 

 the source. Professor Buff then divides his maximum galvanometric 

 deflection, multiplied by 100, by the difference between his initial and 

 his final temperature, and obtains in this way his column of " com- 

 parable values." 



Though begun with a view to conduction, no inference regarding- 

 this subject is drawn from these experiments. We pass, without notice, 

 from conduction to diathermancy, Professor Buff pointing out that his 

 result agrees with that of Magnus in making the denser hydrogen 

 " more diathermanous than the rarefied." Finding, however, in oppo- 

 sition to Magnus, that air showed a similar deportment, being more 

 diathermanous at 760 millims. than at 15 millims., Professor Buff was 

 induced to seek the cause, and if possible avoid the occurrence of so 

 abnormal a result. 



Round the cylinder containing his gases he placed a second and 

 wider one, and filled the space between them with cold water to 

 within 7 millims. of the bottom of his brass vessel. The deflection of 

 the needle fell considerably, and the anomaly of dense air being more 

 diathermanous than rarefied air disappeared. Even here, however, 

 Professor Buff detected currents, or a cause of currents, the heat 

 conducted down the interior cylinder rendering the water warm at 

 the top. Lowering his junction to a depth of 45 millims. below 

 the source of heat, he allowed cold water to enter bis annular space 

 below and to run off above. Here he halted, as if every source of error 

 had been removed. By means of his water jacket he lessened his 

 radiating zone of glass and diminished the area of the heated vertical 

 surface with which his gases came into contact. It is obvious, how- 

 ever, that no contact of the kind ougbt to have been at all permitted. 

 The brass plate, which he regarded as his source of heat, must have 

 accepted almost momentarily the temperature of the water above it ; 

 still, even in the improved experiments, 8 minutes were required to 



