166 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



Hervvig has been led to inquire whether a gaseous layer en- 

 tirely formed of such vapors will not show good conductivity 

 even at low temperatures. He experimented with mercury, 

 dense vapors of which can be used several hundred degrees 

 under white heat. He finds the vapor conductor to resemble 

 that of the voltaic rather than that of a simple metallic con- 

 ductor. There is a peculiar transition resistance which is great 

 in comparison with the hinderances which the current finds 

 within the vapor layer itself, so that the total resistance is in 

 great measure independent of the extent of the vapor layer. 

 The transition resistance is less with increased electro-motive 

 force of battery or strength of current. The vaporization at 

 electrode in the positive mercury surface is increased by the 

 current, an effect similar to that noticed in the voltaic arc, 

 in which, if the electrodes be mercury and platinum, the mer- 

 cury is vaporized only when it forms the positive pole. Using 

 a platinum point and a mercury surface, Herwig finds the re- 

 sistance of the vapor greater when the mercury surface is 

 positive. 12 A, X., 154. 



MOLECULAR CHANGES IN IRON W T ITII VARIATIONS OF TEM- 

 PERATURE. 



Professor Thurston, of the Stevens Technological Institute, 

 has made an important observation in regard to molecular 

 changes produced in iron by the variations of temperature. 

 He comes to the conclusion that, at temperatures above 600 

 Fahr., and below 700, iron conforms to the general law for 

 solid bodies ; that increase of temperature diminishes the 

 tenacity, but increases the ductility and resilience, while de- 

 crease of temperature has the opposite effect. Below 700 

 the tenacity increases with diminishing temperature, while 

 the resilience decreases in a much higher ratio. Between 

 ordinary temperatures and a point somewhere between 500 

 and 600, on the other hand, iron shows a marked deviation 

 from the law, the strength increasingr to the extent of about 

 fifteen per cent, with good iron. The practical result is that, 

 as iron does not lose its power of sustaining steady loads at 

 a low temperature, but greatly loses its power of resisting 

 shocks, the factor of safety in structures need not be increased 

 in the former case where exposure to severe cold is appre- 

 hended ; but that machinery, rails, and other structures which 



