560 Dr. J. T. Bottomley on Radiation of 



At ordinary temperatures its value is given by 



1 = (35 + 3-9 n) (H-4500)10- 8 , 

 where n = frequency, and H= field-strength. 



II. The angle of lag of the bismuth E.M.F. behind the 

 current in the bismuth 



(a) varies with frequency; 



(b) is independent of temperature and field-strength. 



Owens College, Manchester. 



LXIII. On Radiation of Heat and Light from Heated Solid 

 Bodies. By J. T. Bottomley, D.Sc, F.R.S.* 



[PLates V. & VI.] 



THE following paper describes a further instalment of 

 experiments on radiation of heat and light from heated 

 solid bodies, a subject to which I have given attention for a 

 considerable number of years. It is a difficult subject, and 

 it has only been at times that I have been able to make pro- 

 gress in the prosecution of the inquiry. I have treated it 

 from first to last in a purely experimental way. 1 do not 

 feel that there exists at present sufficient experimental in- 

 formation to admit of anything like a complete theory. 

 Indeed the information which we possess is of the scantiest 

 character. 



My aim has been to determine directly the quantity of 

 heat lost from a given surface under given conditions ; and 

 the method by which I have carried out my experiments has 

 been to put the radiating body into a vacuum as complete as 

 I can obtain, and then to measure the energy lost from it in a 

 given time, noting the circumstances. By adopting this plan 

 I avoid determinations of the value of heat-receptors, such as 

 sooted bolometers or sooted thermojunctions ; and I obtain 

 results in absolute measure. 



In a paper published in the Transactions of the Royal 

 Society as far back as 1887 I showed that it is possible 

 to obtain with the Spr en gel-pump a vacuum so good that 

 the effect of convection on loss of heat from a hot body, in 

 so far as that loss is caused by non-condensable gases, 

 ceases to be of importance in comparison with loss due 

 to pure radiation ; in fact, that increasing the vacuum, as 



* Coram umoated bv Lord Kelvin. 



