THE HEAT EMPLOYED. 268 



my attempts to appreciate this effect have proved 

 most unsatisfactory in their results. I mean the 

 results have differed, though the means employed 

 were apparently the same. Then again all en- 

 deavours to institute a comparison between the 

 heat employed in these two methods will be met 

 by similar difficulties. Without entering upon the 

 many failures which I have experienced in such 

 attempts, the general reader will sufficiently com- 

 prehend by the following experiments the difficulty 

 with which this subject is invested. 



With four pounds of good strong charcoal, I 

 raised a drying-tube (Poey long) used for drying 

 black tea, to 220° Fahr. ; the thermometer being 

 placed on the sieve, which Avas elevated 1 8 inches 

 above and immediately over the fire. At this 

 temperature I could bear my hand to rest on the 

 sieve without inconvenience, though water boils 

 at 212°. Indeed I have borne my hand in the 

 same position when the heat has been raised to 

 300° without feeling any sensation of pain. But 

 the iron roasting vessel heated to 180° I could 

 hardly bear to touch. Hence it follows that the 

 iron vessel heated to 180° transmits heat to the 

 leaves, in immediate contact with it, more rapidly, 

 and produces a much greater impression on them, 

 than when placed in the drying-tube, though the 

 heat may be raised to 220° by the conduction of 

 air. So much does the transmission of heat depend 

 on the conducting media. Indeed it is stated by 



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