342 Prof. J. A. Ewing. Contributions to the [June 19, 



observed. Notice is taken of the meaning of the acceleration or 

 polypncea thus evoked, and of its relation to the thermotaxic function 

 suggested. 



Excitation of the corona radiata and the internal capsule resulted 

 in the mapping out of the fibres conducting the above motor effects 

 downwards, and it is suggested that the localisation by previous 

 observers of basal centres for the functions above mentioned is 

 possibly to be accounted for differently in the light of these 

 observations. 



Finally, the results of exploring the floor of the fourth ventricle by 

 excitation are described, so far as intrinsic effects were produced in the 

 larynx. The representation of adduction and abduction movements 

 respectively was thus localised. 



In conclusion, the relations of the various parts of this central 

 mechanism to one another are shortly discussed. 



XIII. "Contributions to the Molecular Theory of Induced 

 Magnetism." By J. A. EWING, F.R.S., Professor of Engi- 

 neering in University College, Dundee. Received June 18, 

 1890. 



As the facts of induced magnetism become better known, increasing 

 interest attaches to molecular theories and increasing difficulty 

 attends the theories that are current. Weber's fundamental con- 

 ception that the molecules of iron or nickel or cobalt are always 

 magnets, and that the process of magnetising consists in turning 

 them from many directions towards one direction, has been 

 strongly confirmed by the now well established fact that 

 there is a true saturation value, a finite limit to the intensity 

 of magnetism, which may be reached or very closely approached 

 by using a strong magnetic force.* Without going further back, 

 to enquire (with Ampere) how the molecules come to be 

 magnets, we may take this conception as the natural starting point 

 of a theory. But when we go on to examine the conditions of con- 

 straint on the part of the rotatable molecules which have been sug- 

 gested to make the theory square with what is known about per- 

 meability, about residual magnetism and other effects of magnetic 

 hysteresis, about the effects of stress, of temperature, of mechanical 

 vibration, and so forth, we find a mass of arbitrary assumptions 

 which still leave the subject bristling with difficulties. Many of the 

 phenomena suggest, for instance, the idea that there is a quasi- 



* Ewing and Low, ' Phil. Trans.,' 1889, A, p. 221. See also fl. E. J. G-. du 

 Bois, 'Phil. Mag.,' April, 1890. 



