.v/A' WILLIAM SIEMKXS, l-.R.S. 441 



may for the present be represented by mF(J). Hence we have 

 Hate of conversion of energy = AT" + mF(t). 



According to Prevost's theory of exchanges, the hot body is 

 its. It receiving radiant energy from the surrounding bodies ; hence 

 the radiant energy is more appropriately represented by A(T" - i n ), 

 where i is the temperature of the surrounding bodies. Similarly 

 it would appear probable that the conduction and convection will 

 depend on the difference of temperature. Hence Rate of energy 

 = A(T n - t n ) + mY(T-f). 



The constants A and m will depend on the nature of the radiat- 

 ing body and on the surrounding medium. 



Although for theoretical purposes it is important to eliminate 

 the conduction and convection, yet in most cases a medium is 

 present, and it has been shown by Mr. Crookes that, within limits, 

 variations in pressure have only a very small effect on the amount 

 of heat lost by conduction and convection. 



I have not as yet been able to make any experiments on the 

 determination of the term wF(T - 1), but it is my intention to 

 make further investigations on this point. I am indebted to Pro- 

 fessor Stokes for suggesting a method which appears to me likely 

 to yield useful results. He proposes to construct a chimney of 

 white paper, and to fix it over the wire through which the current 

 is passing. The chimney will collect all the heated air ascending 

 by convection, and by suitable means its temperature and the rate 

 of flow can be measured, and hence the rate of loss of heat by 

 convection estimated. 



It might be supposed that conducting the experiment in vacua 

 would diminish the convection. According to the original re- 

 searches of Dulong and Petit, the rate of cooling diminished in a 



geometrical progression, whose ratio was ^ 77-, as the pressure 



I'oob 



diminished in a second geometrical progression, of which the 

 ratio was -. Mr. Crookes, in a paper communicated to the Royal 



Society ("Proc. Roy. Soc.," 1880, vol. 31, p. 239) described some 

 experiments on this point, and showed that a diminution of pres- 

 sure from 7CO millims. to 120 millims. had a very slight effect on 

 the convection. From 120 to 5 millims. the effect was somewhat 

 more marked. A reduction of pressure from 5 millims. to '2 

 millimfl., however, produced twice as much fall in the rate of 



