PREFACE. V 



be explained only by the presence of relatively large radiant forces. Noticing, 

 in the course of the work, that the latter, in some mysterious way, almost al- 

 ways acted in the same sense as gravitation, I became much interested in 

 the endeavor to trace the radiant forces to their source and if possible to learn 

 to control them. Much of Chapter XI is given to work of this kind, and among 

 other things the attempt is made to refer the constant of gravitation to the 

 viscosity of the medium in which the needle moves. The most curious re- 

 sults in relation to the radiant forces were obtained by submerging the whole 

 of the gravitational apparatus in a capacious water-bath well stirred, so that 

 the temperature varied but a few tenths of a degree per day. Notwith- 

 standing these apparently ideal conditions, the needle simply drifted and 

 showed no response to gravitation whatever. The best method of reducing the 

 radiation discrepancy in question, thus far found, consists in the exhaustion 

 of the case containing the needle. Results so found came within i per cent 

 of the normal value, but were still in excess. No doubt this is far from pre- 

 cision, but it is a great advance from an error of several hundred per cent of 

 excess. The further development of the method of attack I hope to complete 

 this summer, by making the exhaustion as rigorous as possible. 



In the last chapter I have put together a number of incidental results, bear- 

 ing (as in Chapter VIII) on the breakdown of molecular instabilities evidenced 

 by the peculiar phenomena of residual viscosity. In a similar experiment, 

 showing the magnetic set in iron produced by an electrical current passing 

 through it, we have, as it were, an element of hysteresis. A useful method for 

 the production of two groups of independent fringes, present in the same field, 

 is also given. 



My thanks are due to Miss Rachel Tupper Easterbrooks for efficient as- 

 sistance throughout the editorial work necessary for the preparation of the 

 present report. 



CARL BARUS. 



BROWN UNIVERSITY, June 1921. 



