1875.] President's Address. 93 



"When delicate beams of straw and other materials, carrying pith balls 

 or disks at their ends, were mounted so as to turn as freely as possible 

 within glass vessels from which the air could be exhausted, it was found 

 that at atmospheric pressure the approach of a hot body produced a 

 movement usually indicative of an attraction, real or apparent, between 

 the ball and the body, and that of a cold body a reverse movement. In 

 these motions, however, it is evident that currents of convection play a 

 leading part ; nor does it appear by any means certain that the actions 

 may not be due to these and other well-known causes. The same thing 

 continues when the air is partially exhausted, until, at a certain high 

 rarefaction varying according to circumstances, the motions cease, or 

 nearly so. But on passing this, a new and unexpected phenomenon is 

 revealed, which is exhibited in perfection in chemical vacua, and in the 

 best vacua produced by the Sprengel pump with the improvements which 

 Mr. Crookes has introduced into it. The suspended body is now repelled 

 with striking energy when a source of radiant heat or light is presented 

 to it, or even, if the radiation be powerful, when it is held at some dis- 

 tance, or when the sun's rays concentrated by a lens are thrown upon it. 

 The action has more recently been exhibited by Mr. Crookes in an ex-, 

 ceedingly striking form by means of a horizontal 4-armed fly delicately 

 mounted on a sharp point, and carrying at the ends of the arms pith 

 disks in vertical planes passing through the arms, the disks being 

 blackened on one face, on the same side for all. The motion depends in 

 this case on a differential action on the black and white faces, the black 

 being repelled. 



It is the mystery attending this phenomenon that gives it its great 

 importance. There is evidently some action going, on with which we are 

 not at present acquainted ; and there is no saying what a thorough in- 

 vestigation into the cause of the phenomenon may lead to. 



The Medal was received by Mr. Crookes. 



A Eoyal Medal has been awarded to Dr. Thomas Oldham, F.R.S., for 

 his long and important services in the Science of Geology, first while 

 Professor of Geology in Trinity College, Dublin, and Director of the Geo- 

 logical Survey of Ireland, and chiefly for the great work which he has 

 so long conducted as Superintendent of the Geological Survey of India, 

 in which so much progress has been made that in a few years it will be 

 possible to produce a Geological Map of India comparable to the Geological 

 Map of England executed by the late Mr. Greenough ; also for a series 

 of volumes of Geological Reports and Memoirs, including the ' Palseon- 

 tologica Indica,' published under his direction. 



Professor Oldham is the author of more than twenty Geological Memoirs 

 and Reports, published in the Journal of the Geological Society of Dublin, 

 that of the Geological Society of London, the Journal of the Asiatic So- 



