416 Prof. Magnus on the Influence of the Adhesion of 



depend on a difference in the mode in which the air was admitted, 

 or in its diffusion in the tube, but was due to a difference in the 

 condition of the sides of the tube. For if one tube was slightly 

 blackened on the inside, the difference on forcing in the two 

 kinds of air was greater than before. 



In order that the pile might receive as much heat through 

 the blackened as through the polished tube, the latter had to be 

 strongly deadened by the screen. When now the action of the 

 air in each tube was separately investigated, it was found that, 

 in the one polished internally, the heating effect by the dried 

 and the cooling by the moist air were very strong, but far smaller 

 than in that blackened internally. This was the case although 

 the same quantity of air reached the pile through both tubes, 

 and far more heat entered that blackened on the inside than the 

 polished one deadened by the screen. There could be no doubt, 

 after this experiment, that the inside of the tube had a consider- 

 able influence on this phenomenon. 



That the screen only allowed heat to enter the polished tube 

 through a segment of a circle could not have caused the greater 

 action of moist air in this tube; but in order to remove this ob- 

 jection also, the screen was dispensed with, and by increasing 

 the distance of the cube (the source of heat) the incident quan- 

 tity of heat was diminished. As was to be foreseen, the result 

 was the same. 



The magnitude of the action of which we speak, here and 

 elsewhere, always refers to the total quantity of heat reaching 

 the pile in each case, and is not to be confounded with the de- 

 flection of the galvanometer. 



This total amount of heat was determined by deadening one 

 side of the pile by a screen, while the deflection of the galvano- 

 meter was measured which was produced by the radiation of the 

 other side. Tor greater certainty the other side was also dead- 

 ened, and the mean taken of both values. The deflection was 

 in all cases so great that the scale was insufficient for the obser- 

 vation. To provide for this case a shunt was interposed. Its 

 resistance was to that of the galvanometer in such a ratio that 

 the deflection when the shunt was interposed amounted to only 

 one-sixteenth of the deflection without it. In what follows i 

 do not give the percentage values, because they are not reli- 

 able enough ; for in experiments which depend upon forcing- 

 air into a tube, the same values can never be obtained by repe- 

 tition. Therefore I could only frequently repeat each experiment 

 at different times and under different circumstances, so as to 

 obtain the requisite certainty. But in the experiment mentioned, 

 the cooling in the polished tube amounted to about 3*75 per 

 cent v and in that blackened internally only to 1*4 per cent. 



