128 Prof. H. Lamb. On the Propagation of [June 11, 



each end with a disc of quartz, and through which water was kept 

 circulating to absorb heat rays. The slide on which the hanging-drop 

 cover slip was placed was made of quartz to avoid interception of the 

 ultra-violet rays. The hanging drop thus mounted was then placed on 

 this water-circulating apparatus and the light from the arc was pro- 

 jected through from below upwards, so as to pass through the water 

 and the quartz slide on to the hanging drop. 



We have been unable to find that any of the radiations contained in 

 the spectrum are penetrative to organic substances such as agar, or dead 

 animal or vegetable tissue. Neither can they penetrate living tissues, 

 but we are continuing experiments in this direction to more exactly 

 determine their action. 



The conclusion to be drawn, therefore, is that the bactericidal action 

 of light is almost entirely due to the action of those radiations in the 

 ultra-violet region of the spectrum which are included between the 

 wave-lengths 3287 and 2265. It is, therefore, necessary that any 

 source of light used as a bactericidal agent should be rich in these rays. 



" On the Propagation of Tremors over the Surface ol' an Elastic 

 Solid." By HORACE LAMB, F.R.S. Pteceived June 11, Head 

 June 11, 1903. 



(Abstract.) 



The paper treats of the propagation of vibrations over the surface of 

 a " semi-infinite " isotropic elastic solid, '.<?., a solid bounded only by 

 a plane. For purposes of description, this plane may be conceived as 

 horizontal and the solid as lying below it, although gravity is not 

 specially taken into account. 



The vibrations are supposed due to an arbitrary application of force 

 at a point. In the problem most fully discussed this force consists of 

 an impulse applied normally to the surface; but some other cases, 

 including that of an internal source of disturbance, are also (more 

 briefly) considered. Owing to the complexity of the problem, attention 

 has been concentrated for the present on the vibrations as they 

 manifest themselves at the free surface ; the modifications which the 

 latter introduces into the character of the waves propagated into the 

 interior have accordingly not been examined minutely. 



The investigation may perhaps claim some interest on theoretical 

 grounds, and also in relation to the phenomena of earthquakes. 

 Writers on seismology have naturally endeavoured, from time to time, 

 to interpret the phenomena, at all events in their broader features, by 

 the light of elastic theory. Most of these attempts have been based 



