Magnus on the Condensation of Vapours. 433 



employed, the deviation of the galvanometer was at least as 

 great, and sometimes greater than when the dry or moist air 

 impinged directly on the thermo-electric pile, showing that 

 they condense vapours more readily than the surface of the 

 pile. 



M. Magnus arranged a delicate air thermometer, so that 

 each bulb was surrounded by a small vessel of glass, each 

 glass vessel having a tube proceeding from its neck, immedi- 

 ately over the bulb. On blowing air into one of the glass 

 vessels the thermometer was not affected; but if the air was 

 first dried, then the opposite bulb experienced a cooling 

 action, and became warmer when air saturated with vapour 

 was introduced. The effect was sufficient to produce four to 

 six millimetres difference in the level of the two hmbs of the 

 thermometer. 



A mercurial thermometer divided into half degrees, and 

 sheltered from currents of air, showed under similar circum- 

 stances an effect from 0*2 to 0'3 C. ; and, when the bulb was 

 blackened, the variation reached 0*6 C. 



The rapidity of the effect depends on the thickness and 

 condensing power of the plates. Sal gemmi and other dia- 

 thermic bodies were instantaneously affected; metallic plates, 

 etc., varied according to their conducting power. 



In giving a summary of results, M. Magnus states that 

 different substances, organic and inorganic, wax, paraffin, 

 glass, quartz, mica, gypsum, various salts, metals rough or 

 polished, and also varnished, condense on their surface vapour 

 from the air in which they are placed, and whose temperature 

 is the same as their own. This condensation has a heating* 

 action ; and if drier air is introduced, a portion of the liquid 

 that has been condensed evaporates, and cold is the result. 

 Vapours of alcohol, ether, and other substances produced 

 effects analogous to those of water. In general it may be 

 affirmed that vapours are condensed on solid masses to an 

 extent sufficient to produce appreciable changes of tempera- 

 ture. From this it will appear that on every solid surface ;i 

 layer of vapour always exists, which becomes greater or less, 

 according to the humidity of the air. M. Magnus adds that it 

 cannot be doubted that this film of vapour plays an 

 important part in many actions that occur on the surface of 

 bodies. 



