758 



SCIENCE 



[N. S. Vol. XLIII. No. 1117 



salt solution was measured against both silver ni- 

 trate and potassium nitrate. 



After making certain corrections the above 

 results were obtained from the measured differ- 

 ences of potential of the two electrodes. 



Column three was derived from tables. 



A curve was plotted using column one as ab- 

 scissae and column three as ordinates. 



Notice that the ferric ion does not fall in the 

 position usually given to iron, but in a position 

 near the silver, actually between antimony and 

 mercury. 



The Pressure of Sound Waves: E. P. Lewis. 

 The Passive State of Iron in Nitric Acid: Joseph 



G. Brown. 



It seems evident that the only hope of explain- 

 ing the passive state of metals lies in the detailed 

 study of the process by which some particular 

 metal becomes passive in some particular solution. 

 Accordingly such a study has been made for iron 

 in nitric acid solutions. The E.M.F. of the pri- 

 mary cell: Iron/HNOs solution/concentrated 

 HNOj/platinum, has been measured at room tem- 

 perature from the instant that it was made until 

 it reached a steady state, using eight densities 

 ranging from 1.01 to 1.41, both with the iron at 

 rest and in motion. Observations were made with 

 a low power microscope upon the changes which 

 took place on and around the iron. 



The results show that the ferrous oxide which 

 forms on the iron at the start in all acid densities 

 does not affect the E.M.F. of the cell, but the 

 liquid products do. If the iron is kept at rest in 

 acids up to 1.17 the E.M.F. is increased by the 

 presence of the ferrous nitrate, while in acids 

 denser than 1.17 the E.M.F. is lowered by some 

 other product which forms a bright red liquid film 

 over the oxidized surface of the iron. It is thought 

 that this may be the unstable compound formed 

 by the absorption of nitric oxide by ferrous ni- 

 trate, and the existence of this compound deter- 

 mines the semi-passive state. 



In acids of greater density than 1.25 there is an 

 explosive reaction between the ferrous oxide and 

 the red liquid, after which the iron is in the pas- 

 sive state. The E.M.F. falls very quickly to a 

 minimum and then rises very slowly to an ex- 

 tremely constant value. 



It seems probable that both the ferrous and 

 ferric reactions take place in acids of all densi- 

 ties, but in those greater than 1.25 the ferrous re- 

 action may be quenched by the sudden reaction 



between the ferrous oxide and the red liquid, 

 while the ferric reaction remains. 



There is no indication of the existence of any 

 kind of a film after the passive state is reached, 

 but the gradual change in E.M.F. seems to indi- 

 cate the expulsion of a gas from the iron after the 

 state is reached. 



If the explanation given is correct it allows the 

 interesting conclusion that iron is ' ' active ' ' when- 

 ever the conditions are such that the ferrous ions 

 are formed, but it is " passive ' ' whenever these 

 ions are not formed. This means that iron is es- 

 sentially ferric and the chemical and electrical ac- 

 tion of iron under ordinary circumstances is due to 

 the existence, or formation, of ferrous iron at the 

 surface. The E.M.F. measurements obtained would 

 thus place ferrous iron in the electrode potential 

 series between cadmium and cobalt, which is 

 usually ascribed to iron, while ferric iron falls be- 

 tween antimony and mercury. 



The fact that all those properties of iron which 

 depend upon its cohesion make it more like plati- 

 num than like zinc, and the fact that comparisons 

 of the potential of the same metal in ferrous and 

 ferric salts place the ferrous and ferric iron in 

 these same positions in the series, seem to confirm 

 the conclusion. 



If the significant thing about a valence is a 

 number of electrons, it would seem that the sur- 

 face molecules lost an electron under certain con- 

 ditions but not under other conditions. 



Conductivity of Paints: Raymond B. Abbott. 



A Possible Method for the Detection of Gra/oita- 

 tional Effect on Electrons: Lloyd T. Jones. 



A Formula for Computing a Cohesion Constant : P. 



A. Ross. 



At the Berkeley meeting of the American Phys- 

 ical Society in 1915 a paper was presented by the 

 writer on the "Law of Cohesion in Mercury," in 

 which it was found that the cohesion forces in 

 mercury varied inversely as the sixth power of the 

 distance. A value of the cohesion force was com- 

 puted from the specific heat and coefficient of ex- 

 pansion which integrated on vaporization to a 

 value agreeing well with the latent heat of vapori- 

 zation. 



In the present paper it is shown that from the 

 principle of equipartitiou of energy in a vapor the 

 same cohesion constant may be computed, the 

 formula being 



