430 Mr. G. J. Stoney on the Penetration of 



becomes simply A -7-, or a^ 



dt' AYap,^, (e) 



where A is the area of the surface of the cooling body. 



7. It will be instructive to compare the loss of heat by pe- 

 netration with the quantity which is carried off by convection. 

 To estimate the latter, let 11 be the section of the convection 

 current, p its average density, A6 the average excess of its 

 temperature, and v its velocity. Then the total quantity of 

 heat which will be removed per second by convection will be 



n^<^p^ (?) 



This is to be compared with (e), the expression for the total 

 loss of heat per second by penetration. 



Now, in the cases that occur in laboratory experiments, Aa-p^ 

 is seldom many times larger or many times smaller than flapj 

 but V is always very much larger than v, whence (e) may have 

 a value comparable with (5) while A^^ is very much less than 

 A^ — in other words, when the processions between the opposed 

 surfaces have but slightly different velocities. We learn from 

 this that the escape by penetration may be expected to mani- 

 fest itself as^oon as the Crookes's layer has become in a mode- 

 rate degree compressed *. It is also evident that the coexistence 

 of a convection current will not much affect the escape of heat 

 by penetration, inasmuch as convection currents are sluggish 

 when compared with the promptness with which readjustments 

 are made in Crookes's layers. It is therefore worth while to 

 examine the numerous records of experiments upon the velo- 

 city with which bodies cool in gases, with a view to finding 

 whether instances of the escape of heat by penetration can be 

 found among them. 



Part II. Inte7yretation of Experinrients. 



8. Accordingly I made a search of this kind last year, 

 shortly after the publication of my two papers in the ^ Philoso- 

 phical Magazine,' but without finding any records more to the 

 purpose than those by Dulong and Petit of experiments with 

 hydrogen, which will be cited below ; and as these, taken by 

 themselves, did not seem sufficiently decisive, I postponed 

 publishing further on the subject until I should have leisure 

 to make experiments myself. But before this leisure came, 

 Mr. George F. Fitzgerald met with a brief notice in Jamin's 

 Physique, of experiments by De la Provostaye and Desains, 

 which appeared to him to contain observations on the pene- 



* Hence also thermal experiments maybe expected to explore Crookes's 

 layers witli more sensitiveness than contrivances for manifesting the me- 

 chanical force which is also present. 



