264 THERMOMETRICAL TEST UNSATISFACTORY. 



Brande* that iron heated to 120° is painful to the 

 touch, but that air may be heated to 250 and 300° 

 without being painful to our organs. He moreover 

 states that Sir F. Chan trey had a room for drying 

 his models for casts heated to 300° Fahr., which 

 the workmen were accustomed to enter in the 

 ordinary course of business. Thus if we take 

 the difference of temperature given by the ther- 

 mometer as indicative of the amount of heat im- 

 pressed on the leaves in these two methods, it is 

 obvious we shall be led to form very fallacious 

 conclusions. 



Moreover, in these experiments much depends 

 on the strength of the charcoal, the state of the 

 atmosphere, force of wind, and other conditions, as 

 such experiments can only be conducted with 

 convenience in open sheds, where there is a free 

 current of air. Much also depends, I believe, on 

 the size, form, depth and thickness of the roasting 

 vessel as affecting its radiating powers and focus 

 of heat, not to speak of the many sources of error 

 to which all such experiments are exposed. 



If any satisfactory comparisons can be instituted 

 on this subject, they are only to be obtained by the 

 joint efforts of a good practical chemist, and a 

 skilful manipulator, with an abundance of leaves 

 at their disposal, and with all the necessary imple- 

 ments at hand. 



* Manual of Chemistry, p. 159. 



