RADIATION IN ABSOLUTE MEASURE. 
445 
was to avoid these uncertainties that I was led to abandon the far more manageable 
(for these experiments) glass envelope, and to take in its place a copper envelope 
blackened on the inside. 
The next series of experiments which I desire to describe was undertaken for 
the purpose of tracing the rates of emission of heat at one fixed difference of 
temperatures between cooling body and surroundings, but at different pressures; 
commencing with ordinary atmospheric pressure, and passing to the highest vacuum I 
could command. The experiments, of which the results are given in the Tables, pp. 447, 
448, were made with the apparatus of fig. 1. The slider was set at a particular point of 
the scale ; and, by adjusting with the rheostat, the current was found which w r as 
required to maintain the temperature in the wire corresponding with the fixed 
position of the slider, while the pump was worked and the vacuum gradually 
produced. A complete series of readings was obtained from a common atmospheric 
pressure downwards for the temperature 408° C. ; and a partial series for the tempera¬ 
ture 505° C., commencing with the pressure '094 mm. This kind of experimenting, as 
I have already remarked, only recently became possible, or at all events practicable. 
It requires a current galvanometer of great sensitiveness, and easily readable in a 
continuous way, qualities which have only now been supplied by the invention and 
perfecting of Sir W. Thomson’s current galvanometers. It is, however, an extremely 
convenient and satisfactory method. 
An inspection of the Table shows the smallness of the falling away of the rate of 
loss of heat between full atmospheric pressure and pressures of 10 mm., 5 mm., or 
even 1 mm. ; and the very rapid fall at pressures slightly lower than 1 mm. This has 
already been shown for temperatures below 100° C. by the experiments of Kundt 
and Warburg and of Crookes, and is in accordance with the theory and experiments 
of Maxwell, which show that heat conduction in gases is independent of the pres¬ 
sure so long as the density is such that the average length of free path of the mole¬ 
cules remains small in comparison with the dimensions of the containing vessel. The 
numbers given in my Tables are, as in all other cases throughout this paper, in absolute 
measure. The curve (see Plate 26) shows graphically the relation between pressure 
and energy lost at the temperature 408° C. The line of abscissas represents the 
pressure, wdiile the ordinates represent the energy lost. 
Carrying on the experiment as described above, I was able, as the vacuum gradually 
improved, to reach an interesting condition of matters in which further rarefaction of 
the space produced no further diminution of the rate of loss of energy from the wire. 
This was reached in the following way. The radiation wire was kept from the morning 
of March 28 till the morning of March 30 at a moderately high vacuum, and 
with the current incessantly flowing through it. During parts of March 28 and 29 
experiments on radiation were being carried on, and during the evenings and 
nights of the 28th and 29th the current was raised so as to maintain the wire at a 
temperature very much higher than that at which I proposed to experiment. On 
