54 Emission and Transmission of Heat 



other through a sheet of metal; and we may admit that, within 

 the limit of thicknesses generally employed, the material and thick- 

 ness of the metal are without sensible effect; but if in certain cases 

 there should be a great advantage in increasing the rate of this 

 transmission, even at the expense of doing work for this purpose, 

 it could be effected by a stirring which would very rapidly renew 

 the layers of liquid in contact with the plate. 



All of the preceding remarks suppose both surfaces of the 

 plates to be bathed by liquids, and should therefore be only ap- 

 plied to heating of liquids by liquids, or by steam; for steam in 

 condensing on the surfaces of the plates wets them and all takes 

 place as if the heating were done by a liquid. But when liquids 

 are heated by gases, and when gases are heated by other gases is 

 it still the same ? It is this which we will examine next. 



835 . Consider first the heating of liquids by gases, this is for 

 example, the case with steam boilers, at least for that part of a 

 boiler which does not receive the radiation from the grate. I 

 have made no direct experiments on this subject, but the results 

 of practice leave no room for doubt that if the nature and thick- 

 ness of the metal have any influence it is only very small; for it 

 has been recognized for a long time that boilers of cast iron, of 

 copper, and of wrought iron, of the same dimensions but of widely 

 differing thicknesses, give sensibly the same results under the 

 same circumstances. It is a fact upon which all engineers agree. 

 One may easily account for it: when the thickness of metal in- 

 creases or its conductivity decreases, the temperature of its outer 

 surface rises; a well proven fact since in cast iron boilers the ex- 

 terior often becomes red, and as to wrought iron boilers, the 

 change of shape which they experience from the heat increases 

 with their thickness. Now since the quantity of heat transmitted 

 increases as the exterior temperature increases, it is easy to see 

 that the effect of the nature and thickness of the metal is but 

 slight. 



This has been proven further by recent experiments of M. 

 Boutigny; who evaporated water in silver capsules of the same 

 form and exterior dimensions but of very different thicknesses; 

 the quantities evaporated under the same conditions and in the 

 same time were precisely the same. 



