SENSITIVE FLAMES, ETC. 191 



heat either boiling water, or a heated platinum spiral, or 

 the flame of a lamp. This heat he transmitted through a 

 long experimental tube, the ends of which were closed by 

 plates of rock-salt, and which tube could be exhausted by 

 means of an air-pump. The radiation from the source 

 passing through the exhausted tube and plates of rock-salt 

 practically unhindered, fell upon the blackened face of a 

 thermopile. This pile was associated with a galvano- 

 meter, and the deflection produced on the galvanometer 

 by the heating of the anterior face of the pile was 

 compensated by warming the posterior face of the pile, and 

 so the needle of the galvanometer was brought to zero. 

 If now into this exhausted tube an elementary gas were 

 introduced, such as oxygen or hydrogen or nitrogen, it was 

 found that little or no difference in the flux of heat through 

 the tube was indicated ; whereas if, instead of an elemen- 

 tary gas, a compound gas, such as olefiant gas or ammonia, 

 or any other of the many compound gases, were used, a very 

 great diminution in the flux of heat was perceived. It was 

 greatest in the case of the most complex gases, and least 

 in the case of the least complex gases. Professor Tyndall, 

 however, went much farther than this. He examined the 

 radiation as well as the absorption of these gases the 

 radiation obtained by heating the gas and allowing it to 

 stream up in front of the thermo-electric pile, and the 

 result showed that the radiation from the elementary 

 gases was feeblest, the radiation from the compound gases 

 being found most abundant. But by another method he 

 examined the subject with still greater care. Doing away 

 with all sources of heat, and dividing the experimental tube 

 into two parts, separated by a plate of rock-salt,, he ex- 

 hausted both chambers. Into the chamber most distant 

 from the pile he now allowed a gas to stream. When an 

 elementary gas was thus allowed to enter the exhausted 

 tube, the collision of its particles against the side of the 

 tube caused a conversion of motion into heat. The par- 

 ticles of gas were warmed, and radiated their heat through 

 the exhausted portion of the tube and to the pile. This 

 Professor Tyndall termed " dynamic radiation." If now a 

 compound gas. were allowed to enter this distant portion 

 of the tube, after it had been once more exhausted, a still 

 greater radiation was perceived, just as in the case when the 



