_G, Jounstone Stoney on the Penetration of Heat across Layers of Gas. 25 
the theoretical consideration that the members of the swift procession of 
molecules of hydrogen will be more diverted from their course if some of the 
molecules they encounter are the heavier molecules of other gases. 
18, Several phenomena observed by Grove, Tyndal, Magnus, and others, have 
been attributed to copious conduction of heat through hydrogen. May we not 
with more probability refer them to the very remarkable power which hydrogen 
possesses of allowing heat to leak away by penetration? From the dynamical 
theory of gases it seems improbable that any gas can possess true conducting 
power in a high degree, and every observer must have been struck by the violence 
of the first chilling effect in some of these experiments, and the unflagging energy 
with which it is maintained, a promptness and persistence characteristic of 
penetration, but quite unlike the moderate initial effect and diminished subsequent 
progress which we should expect from conduction. 
19.* [One of the most striking of these experiments is that by Sir William 
Grove which exhibits the cooling effect of hydrogen on a wire rendered incandes- 
cent by the passage of an electric current; and according to the hypothesis here 
presented it ought to be possible to repeat this experiment in ordinary atmospheric 
air by bringing the incandescent wire sufficiently close to a cool object. This very 
instructive experiment was proposed by my son, Master Gerald Stoney, and has been 
‘successfully performed by him. He first passed the wire through a glass tube drawn 
out sufficiently thin. The effect could then be seen, but it was evanescent because 
the glass became rapidly heated. No doubt if the tube had been surrounded by a 
water jacket, the experiment might have been made in this way satisfactorily. 
But he made it in an equally permanent form and with greater ease, by simply 
bringing the incandescent wire close to a tin can containing water, to which the 
heat leaked away abundantly from the wire when the intervening stratum of air was 
sufficiently thin. The effect is best seen when the wire is of a dull red, on account of 
the ease with which the eye detects the difference between dull red and darkness. It 
then becomes conspicuous when the interval is a millimetre, and can be perceived 
when the interval is considerably more. In this experiment the can was at a slightly 
higher temperature than the room. On this account, and because a small part of 
the radiated heat was reflected back on the wire by the tin, the loss of heat by 
radiation was less than when the can. was away. Moreover, the convection 
current was enfeebled by being both cooled and obstructed by the neighbouring 
obstacle. Hence the true loss of heat by penetration must have been in excess 
of that which manifested itself. ] 
20. Another phenomenon which admits of explanation by the theory developed 
here and in my former papers is one which is said to have caused the bursting of 
steam boilers, which is familiar to us as the way in which a laundress tests the 
* Section 19 was re-written June 12, 1877. 
