34 



Prof. Tyndall on the Relation of 



In confirmation of the re- 

 sults obtained when my tube 

 was stopped by plates of rock- 

 salt, I have recently made the 

 following experiments with a 

 tube in which no plates were 

 used. S is the source of heat, 

 and ST the front chamber 

 which is usually kept ex- 

 hausted, being connected 

 with the experimental tube 

 at T. This chamber is now 

 left open. A B is the expe- 

 rimental tube, with both its 

 ends also open. P is the 

 thermo-electric pile, the an- 

 terior face of which receives 

 rays from the source S, while 

 its posterior surface is warmed 

 by the rays from the com- 

 pensating cube C At c and 

 d are two stopcocks — that at 

 c being connected with an 

 india-rubber bag containing 

 air, while that at d is con- 

 nected with an air-pump. 



My aim in this arrange- 

 ment was to introduce at 

 pleasure, into the portion of 

 the tube between c and d } dry 

 air, the common laboratory 

 air, or air artificially moist- 

 ened. The point c, at which 

 the air entered, was 18 inches 

 from the source S ; the point 

 d, at which the air was with- 

 drawn, was 12 inches from 

 the face of the pile. By 

 adopting these dimensions, 

 and thus isolating the cen- 

 tral portion of the tube, one 

 kind of air may with ease 

 and certainty be displaced by 

 another without producing 

 any agitation either at the 

 source on the one hand, or 

 at the pile on the other. 



§ 



W 



=$ 



6C 



