444 
MR. J. T. BOTTOMLEY OX THERMAL 
Pt 3 . (Enclosure at 0° C.) 
t 
C 2 R/Js 
Em 
o 
16 
9 75 x 10- 1 
60-9 x 10~ 6 
38 
257 
67'6 ,, 
94 
78-7 „ 
83-7 „ 
228 
335'5 „ 
147 
403 
1182 
293 
585 
3157 
540 
B 5 , Wire experimented on by myself. (Enclosure at 15°.) 
1 
t 
C 2 R/Js 
Em 
O 
302 
186-7 x 10~ 4 
65-05 x lO- 6 
425 
493-5 „ 
120-3 
613 
1689 ,, 
282 
744 
3918 
537 
806 
5163 
653 ,, 
From these Tables it will be seen that of the two polished platinums, Ptj and 
Pt 2 , the latter has a somewhat greater emissivity; while in the case of the 
blackened platinum, Pt 3 , the emissivity is very much greater, being at 585° 0., 
the highest temperature to which the blackened platinum was carried in these 
experiments, about four times that of the polished platinums. The numbers which 
I have quoted from one of my own experiments for comparison give an emissivity 
for polished platinum, with the vacuum of July 1 and 2, 1886, much higher than 
that found by Schleiermacher. This may be due partly to difference in the polish 
of the surfaces, as I find that a very slight difference in this respect produces a very 
great effect. It may be, also, that the vacuum at which the determinations of 
Schleiermacher were made was of a higher order than mine of July 1 and 2, though 
I do not think this probable.* I think, however, it is most likely that the true 
explanation of the greater part, at least, of the difference lies in the fact that 
Schleiermacher surrounded the wire with a glass envelope. It is impossible to 
estimate with a non-diathermanous and badly conducting substance like glass how 
much heat may be returned to the wire from the polished walls of the tube; and it 
found, with the exception of those given by Macfarlane, a few given by Pictet, and one or two by 
Kundt and Warburg, expressed in units of heat or units of energy emitted, under definite circumstances, 
per square centimetre per second. 
* Schleiermacher used the Bessel-Hagen pump. My experience of this pump does not lead me to 
feel confidence in it as being capable of producing a very high vacuum. My reasons for this statement 
were given to the British Association, September, 1886, in a paper “ On an Improved Mercury Pump. 
