Unity of the General Forces of Nature. 211 



Fis:. 11. The same; external view of animal contracted 

 in alcohol. 



Enlarged about one half. 



Fiff. 12. Pellicula convexa Martens. 



Centrals and laterals ; copied from Heynemanu, I. c, pi. iv, fig. 5. 



Fig. 13. The same ; marginal tooth. 



Fig. 14. The same ; jaw. 



Fig. 15. Hemphillia glandidosa. 



Extreme marginal teeth. 

 Fig. 16. The same ; first marginal teeth. 

 Fig. 17. The same ; central and lateral teeth. 



XVII. — Essay ujjon a JVecessary Limitation of the Doc- 

 trine of the Unity of the General Forces of Nature. 



BY PROFESSOR BENJAMIN N. MARTIN, 



of the University of the City of New York. 



Read Nov. 6, 1872. 



The great and characteristic doctrine of our modern 

 physics is that which affirms the unity and the convertibility 

 of the forces of nature. Varied and multiform as are the 

 diflfused agencies of the physical universe, it is found that 

 they are fundamentally one ; and the proof of this oneness 

 is furnished by the fact that they are all convertible into one 

 another. On the one hand, electricity, magnetism and gal- 

 vanism, — on the other, light and heat, may be made to 

 produce each other. One of these forms of force can dis- 

 appear only by giving birth to another ; and the sum of 

 them all is ever the same. Under certain conditions, gal- 

 vanic electricity will manifest itself as light and heat, and 

 heat will develop electricity again. Each is a form of 

 motion convertible into the other. Moreover, they sustain a 

 common relation to the motion of the masses of matter 



