Geology and Mineralogy. 351 



l mm apart, the repulsion in question is of the order of *5 dyne and 

 therefore equivalent to a pressure of *0015 dynes/cm or roughly 

 10~ 9 atmospheres. It is in excess of any electric repulsion due to 

 the absolute voltaic potential of the discs. The suspended plate 

 reaches its position of equilibrium gradually, the motion progress- 

 ing at a retarded rate through infinite time, in a way character- 

 istic of the viscosity of the film of air between the plates. 



I have estimated the intensity of the force both from the 

 repulsions of a vertical plate suspended from the horizontal 

 pendulum on opposite sides of a fixed parallel identical plate ; 

 also by charging pairs of plates to a given difference of potential 

 for a given distance apart. So far as can be seen, the repulsion 

 is caused by the condensation of air on the surface of the plates 

 by molecular and not by gravitational force (which is too small). 

 Hence, the method employed should enable the observer to find 

 the density of the concentration in terms of the distance from the 

 plate and the law of attraction of the plate in terms of distance, 

 within the small distances in question. In other words, a method 

 for the direct investigation of molecular force is here apparently 

 given. 



Brown University, Providence, E. I., Feb. 24. 



II. Geology and Mineralogy. 



1. Third Annual Report of the Director of the Bureau of 

 Mines, Joseph A. Holmes, for the fiscal year ended June 30, 

 1913. Pp. vii, 118. Washington, March, 1914.— The Bureau of 

 Mines was established by act of Congress in 1910, and began its 

 work on July 1st of that year, taking over the investigations 

 lying in its field which had been carried on previously by the 

 17. S. Geological Survey. A new act of February 25, 1913, estab- 

 lished a bureau of " Mining, Metallurgy, and Mineral Technology," 

 widening the scope of the Bureau and enlarging its purposes and 

 more clearly defining its work. This enlargement was the grati- 

 fying result of a general demand throughout the country; but it 

 is to be regretted that as yet no direct provision has been made 

 for carrying the larger plans into effect. 



The record of work accomplished during the three years of the 

 existence of the Bureau is a notable one and deserves fuller treat- 

 ment than can be given here. The general subject of safety in 

 mining, particularly with reference to investigating the causes 

 and the means of prevention of coal-mine explosions, and the 

 safe-guarding of the miners, was the one to which the energies of 

 the Bureau were first directed, and in which most has been 

 accomplished. Mine-rescue and first-aid work have also been 

 pushed forward with vigor, and it is satisfactoiy to learn that 

 the percentage of deaths per thousand employees due to accidents 

 in the coal mines have been reduced from 4-88 in 1907 to 3-27 in 



