II THE PROGRESS OF SCIENCE 107 



and nebulae which afford sufficient light to affect 

 the eye, or the photographic plate, of the inquirer. 



The mathematical and experimental elucidation 

 of the phenomena of electricity, and the study of 

 the relations of this form of energy with chemical 

 and thermal action, had made extensive progress 

 before 1837. But the determination of the in- 

 fluence of magnetism on light, the discovery of dia- 

 magnetism, of the influence of crystalline structure 

 on magnetism, and the completion of the mathe- 

 matical theory of electricity, all belong to the 

 present epoch. To it also appertain the practical 

 execution and the working out of the results of 

 the great international system of observations on 

 terrestrial magnetism, suggested by Humboldt in 

 1836 ; and the invention of instruments of infinite 

 delicacy and precision for the quantitative deter- 

 mination of electrical phenomena. The voltaic 

 battery has received vast improvements ; while 

 the invention of magneto-electric engines and of 

 improved means of producing ordinary electricity 

 has provided sources of electrical energy vastly 

 superior to any before extant in power, and far 

 more convenient for use. 



It is perhaps this branch of physical science 

 which may claim the palm for its practical fruits, 

 no less than for the aid which it has furnished 

 to the investigation of other parts of the field of 

 physical science. The idea of the practicability of 

 establishing a communication between distant 



