LIFE AND WORK OF LORD KELVIN— THOMPSON. 765 



most recondite phenomena — the mysteries of magnetism, the marvels 

 of electricity, the difficulties of crystallography, the contradictory 

 properties of ether, the anomalies of optics. While Thomson had 

 been seeking to explain electricity and magnetism and light dynamic- 

 ally, or as mechanical properties, if not of matter, at least of mole- 

 cules, Maxwell (the most eminent of his many disciples) had boldly 

 propounded the electromagnetic theory of light and had drawn all 

 the younger men after him in acceptance of the generalization that 

 the waves of light were essentially electromagnetic displacements in 

 the ether. Thomson had never accepted Maxwell's theory. It is true 

 that in 1888 he gave a nominal adhesion, and in the j^reface which, 

 in 1893, he wrote to Hertz's Electric Waves, he himself uses the 

 phrase " the electromagnetic theory of light, or the undulatory theory 

 of magnetic disturbance." But later he withdrew his adhesion, pre- 

 ferring to think of things in his own way. Thomson's Baltimore lec- 

 tures, abounding, as they do, in brilliant and ingenious points, and 

 ranging from the most recondite problems of optics to speculations 

 on crystal rigidit}^, the tactics of molecules and the size of atoms, leave 

 one with the sense of being a sort of protest of a man persuaded 

 against his own instincts and struggling to find new expression of his 

 thoughts so as to retain his old ways of regarding the ultimate 

 dynamics of physical nature. 



One characteristic of all Lord Kelvin's teaching was his peculiar 

 fondness for illustrating recondite notions by models. Possibly he 

 derived this habit from Faraday; but he pushed its use far beyond 

 anything prior. He built up chains of spinning gyrostats to show 

 how the rigidity derived from the inertia of rotation might illustrate 

 the property of elasticity. The vortex-atom presented a dynamical 

 picture of an ideal material system. He strung together little balls 

 and beads Avith sticks and elastic bands to demonstrate crystalline 

 dynamics. On the use of the model to illustrate physical principles 

 he spoke as follows at Baltimore : 



My object is to show how to make a mechanical model which shall fulfill the 

 conditions required in the physical phenomena that we are considering, what- 

 ever they may be. At the time when we are considering the phenomena of 

 elasticity in solids I shall want a model of that. At another time, when we 

 have vibrations of light to consider, I shall want to show a model of the action 

 exhibited in that phenomenon. We want to understand the whole about it ; we 

 only understand a part. It seems to me that the test of " Do we or do we not 

 understand a particular subject in physics?" is "Can we make a mechanical 

 model of it?" I have an immense admiration for Maxwell's mechanical model 

 of electromagnetic induction. 



And again Lord Kelvin says : 



I never satisfy myself until I can make a mechanical model of a thing. If I 

 can make a mechanical model, I can understand it. As long as I can not make 

 a mechanical model all the way through I can not understand it. 



