90 Prof. Minchin on the Action of Electromagnetic 



Note by Prof. William Kamsat. 



This paper reached rny hands from Professor MendeleefT 

 with the request that it should be published in the Philo- 

 sophical Magazine. Professor Fitz Gerald has had the kindness 

 to see it through the press, and has made some alterations 

 which were necessary from thermodynamical considerations. 

 It is unfortunate that Professor Kraevitch has made use of 

 data in verifying his equations which are, to say the least, 

 not the best attainable. But the main fact, that he has devised 

 a rational formula which makes possible the calculation of 

 vapour-pressures, by means of data derived from other physical 

 constants, is of great importance. 



V. The Action of Electromagnetic Radiations on Films con- 

 taining Metallic Powders. By Professor G. M. Minchin, 

 M.A* 



AT the last Edinburgh meeting of the British Association 

 attention was directed by Dr. Dawson Turner to a dis- 

 covery made by M. E. Branly, as to the effect produced on a 

 glass tube filled with copper filings through the extremities of 

 which tube are inserted two wires which dip into the filings 

 (without, of course, touching each other inside the tube). The 

 filings form, apparently, a continuous column of metal, the 

 tube when held up to the light being opaque. Supporting 

 the tube in a horizontal, or any other, position, and connect- 

 ing its terminal wires with a galvanometer and a voltaic cell 

 in a continuous cireuit, the chances are that no "current what- 

 ever will be indicated by the galvanometer. If such a current 

 happens to exist, a very slight tap given to the tube or to its 

 support will destroy the conductivity of the column of filings, 

 and no current passes. Another such tap will cause the 

 column to conduct, and so on. 



At first sight it seems strange that such a metallic column 

 should be a non-conductor ; but, of course, when we see the 

 result we can admit that the contact between every particle 

 and its neighbours is of the microphonic kind, and thus that 

 the whole column may be an extremely bad conductor. 



The identity of the state of affairs in the tube with that in 

 a microphone may be perceived by inserting a telephone in 

 the circuit, and then we shall find that slight mechanical dis- 

 turbances communicated to the column of filings will be 



* Communicated by the Physical Society : read November 24, 1893, 



