of Porcelain, Gilded and not Gilded. 245 



eminent degree the power of reflecting the invisible rays 

 or undulations which all objects in nature send ofF con- 

 tinually and in all directions from their surfaces in con- 

 sequence of that peculiar motion of their particles which 

 constitutes their temperature. , 



Hence it appears that vessels having a metallic sur- 

 face on the outside must be well adapted for preserving 

 the temperature of the substances which they contain, 

 whether that temperature be high or low, warm or cold. 



I am far from maintaining that the sort of material 

 of which the vessel is made, and the thickness of its 

 walls, are matters entirely indifferent, provided that the 

 outer surface be covered with a thin metallic layer which 

 is clean and bright. I am aware that neither heat nor cold 

 can be communicated or propagated through the walls 

 of a vessel, or of any other solid body, instantaneously, 

 and that this communication takes place more quickly 

 in some substances than in others, more quickly 

 through a thin wall than through a thick wall of the 

 same material ; and it is evident that this difference 

 must necessarily exert an influence on the rapidity of 

 the change of temperature of the vessel and of the 

 liquid it contains, whatever be the nature of the exter- 

 nal surface of the vessel. 



For example, as porcelain is a worse conductor of 

 heat than gold or silver, a vessel of given form and 

 dimensions, made of porcelain and well gilded on the 

 outside, if filled with warm water, would cool rather 

 more slowly in the air, or even in a Torricellian vacuum, 

 than another vessel of the same dimensions made of 

 gold or silver, and filled with water of the same tern- 



D ' 



perature ; but if the vessels were exposed at the same 

 time to a strong and very cold current of air, or were 



