230 Anniversary Meeting. [Nov. 30, 



logical authority on the structure and distribution of the Graptolitida?. 

 For some years past he has been engaged in a laborious study of the 

 Silurian and Cambrian rocks of the middle of England, the detailed 

 publication of which is awaited with much interest by geologists. 



Professor Riicker, F.R.S. (Royal Medal). 



In conjunction with Professor Remold, Professor Riicker carried 

 out an important series of researches (extending over ten years) on 

 the electric resistance and other physical properties of liquid films, in 

 the course of which the fact was established that the black part of 

 a soap film in equilibrium has a uniform or nearly uniform thick- 

 ness of 11 or 12 micromillimetres, and that there is an abrupt 

 augmentation across its border to a thickness of about 30 or 40 micro- 

 millimetres in passing to the coloured portions. This, considered in 

 connection with the well-known sudden opening out of the little black 

 areas in an ordinary soap-bubble, proves a minimum of surface-tension 

 for some thickness between 10 and 50 micromillimetres, which, in the 

 ordinary soap-bubble unmodified by Reinold and Riicker's electric 

 current, is temporarily balanced in virtue of the abrupt change of 

 thickness, a proposition of fundamental importance in the molecular 

 theory, implying the existence of molecular heterogeneousness. 



In theoretical calculations connected with the compounding of 

 dynamos and motors to produce constant potential difference, constant 

 current, or constant speed, electricians did not see their way to obtain 

 results of a sufficiently simple character to be of use in practice, if 

 they employed a function of the current which fairly represented the 

 magnetism. They were, therefore, compelled to assume in such 

 calculations that the magnetism was a linear function of the current, 

 although it was well known that this was very far from being true 

 when the current was large. Professor Riicker, however, developed 

 a simple method of attacking such problems, and showed how the 

 magnetic saturation of the iron might be taken into account, and 

 a comprehensive solution of the general problem of compounding 

 dynamos and motors obtained in a workable form. Professor Riicker's 

 paper containing his investigation, and which will be found in the 

 ' Proceedings of the Physical Society,' is a most valuable contribution 

 to the theory of direct-current dynamos and motors. 



Professor Riicker has, with the co-operation of Professor Thorpe, 

 completed a Magnetic Survey of the British Isles (1884-89), which, 

 independently of its great value in investigations of the distribution 

 of the earth's magnetism, and the changes to which it is subject, is 

 specially remarkable for the exhaustive discussion of the observations 

 in reference to regions of local magnetic disturbance, and their rela- 

 tion to the geological constitution of the earth's crust in the neio-h- 



