1849] WRITINGS OF JOSEPH HENRY. 289 



If the force is much less, the effect does not take place. 

 Several j^ears ago I placed a small quantity of water in a 

 retort, and joined the beak of this to the open beak of 

 another retort filled with air. The retort containing water 

 was placed within a room kept constantly at a mean tem- 

 perature of about 65°, while the body of the other retort was 

 without a window, and constantly at a mean temperature of 

 not more than 40°. Though the apparatus was suffered to 

 remain thus, during a whole winter, not a single drop of 

 water passed over. The force of diffusion due to the differ- 

 ence of tension in the two retorts was in this case too small 

 to overcome the resistance of the atoms to a passage between 

 each other. 



ON THE RADIATION OF HEAT. 



(Proceedings of the American Philosophical Society, vol. v, p. 108.) 

 October 19, 1849. 



Professor Henry communicated some experiments which 

 he had made upon the subject of the radiation of heat. It 

 occurred to him, from the constitution of the atmosphere, 

 that if the air were a good radiator of heat, the higher tem- 

 peratures below and the lower above could not be permanent. 

 By placing a thermo-multiplier before a flame, interposing 

 a screen of wood with a hole through it, radiation from the 

 flame was perceived becoming less as the flame was lowered, 

 and still existing, though in small quantities, from the heated 

 air above the flame. He also repeated the experiments upon 

 the radiation of heat from flames. The radiation of heat 

 from the flame of hydrogen is but small, as is its radiation 

 of light. This radiation is much increased by placing a 

 solid in the flame. This is in accordance with Count Rum- 

 ford's assertion, that clay balls placed in the fire increased 

 the amount of heat. 



Professor Henry also mentioned some experiments which 

 he had made some years ago upon the reflection of heat from 

 ice with a concave mirror of that substance. 

 19 



