( 132 ) 



branches with thick foliage collected by Mr. K. A. R. Bosscha, on 

 which Dr. Valeton's addition to Gamble's diagnosis is based, I can 

 now conform this amplification of Gamble's diagnosis, sent me on 

 May 12 th by Dr. Valkton, thanks to supplementary material received 

 to-day (June 27 th 1908) from Dr. A. Pulle (Utrecht) and at once 

 forwarded to Mr. Gamble. In order to complete the diagnosis of 

 Oreiostachys, and to settle the question of the further probable synonymy 

 of this interesting species, a question raised by Mr. Gamble and myself 

 in the Proceedings of April 24 th there now only remain as desiderata 

 the collection of stem-sheaths and the examination of the authentic 

 specimens of Bambusa elegantissima Hassk. and Schizostachyum 

 elegantissimum (Hassk.) Kurz, which so for have not been found, either 

 by Dr. Valeton at Buitenzorg, or by myself at Leiden or Utrecht. 



Leiden, June 27 th 1908. 



Physics. — "On the law of molecular attraction for electrical 

 double points". By Prof. J. D. van der Waals Jr. (Commu- 

 nicated by Prof. Dr. J. D. van der Waals). 



Several physicists have already urged the supposition that the 

 molecular attraction results from the electric forces exercised by 

 electrically charged particles which are contained in the molecule. 

 One of the simplest suppositions we can make in trying to explain 

 the molecular action from an electrical origin is that the molecules 

 will behave as electrical double points. This has, in fact, been 

 assumed by Mr. Reinganum 1 ) and by Mr. Sutherland 3 ). 



As the formula for the action between two electrical double points, 

 which is the same as that for the action between two magnetic 



molecules 8 ), contains — as a factor, — r representing the distance 



between the two double points, — these physicists concluded that 

 the molecules would attract one another with a force proportional 



to — . The opinion that the electron-theory supports the supposition 



of a molecular attraction proportional to — has accordingly been often 



r 



advanced. 



i) M. Reinganum, Phys. Zeitschr. 2, 241 (1901); Drudes Ann. 10, 334 (1903). 



2) W. Sutherland, Phil. mag. (6) 4, 625 (1902). 



3 ) Gf. J. G. Maxwell, A treatise on electr. and magn. Art. 387. 



