248 HEAT EMPLOYED IN THE 



iron vessel in the process of Ta-ching, fall down 

 immediately to its extreme edge and the sides of 

 the brick work, which of course are less heated. 

 Positive contact, therefore, as in some analogous 

 cases, may not take place, or be only momentary. 

 It is observed by Brande, that " if a little water 

 be carefully dropped into a platinum crucible, 

 heated nearly to redness, the water lies quietly 

 upon the metal, so long as it is prevented coming 

 into perfect contact with it by the interposed layer 

 of steam ; but as soon as the crucible cools down 

 so as to admit the water to touch it, it instantly 

 boils off."* In this example we seem to have 

 strong evidence of the nonconducting power of 

 steam ; but as here the almost quiescent state of 

 the water is a necessary condition to the success 

 of the experiment, whereas the tea leaves are kept 

 in constant motion, it may be, that the true cause 

 of their not burning is due to a rapid conduction 

 of heat, which takes place through every particle and 

 pore of the leaf, by the sudden and copious gene- 

 ration of steam which is thus called into action ; as 

 in a well known and familiar experiment, heat by 

 rapid conduction may be transmitted innoxiously 

 through a sheet of paper, whereas if slowly 

 conducted the paper ignites. 



But whether one or both of these causes may 

 operate, or to whatever cause the effect may be 



* Manual of Chemistry, p. 158. 



