176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



that there must be a close connection between the atom of matter 

 and its electrical charge. Maxwell and Helmholtz suggested that 

 the results were simply explained by supposing that electricity was 

 atomic in nature. This conclusion is now definitely established, and 

 the positive charge carried by the hydrogen atoms in the elec- 

 trolysis of water is believed to be the fundamental unit of electrical 

 charge. This charge is equal to and opposite to the charge carried 

 by the electron. Any charge of electricity, however small or large, 

 must be expressed by an integral multiple of this fundamental unit 

 of electricity. The actual value of this unit charge has been meas- 

 ured by a great variety of methods and concordant results. One 

 of the most detailed and accurate investigations of this important 

 constant has been made by Prof. Millikan, of the University of 

 Chicago. 



OBJECTIONS TO ATOMIC THEORY. 



We have so far implicitly assumed that the great majority of 

 scientific men now regard the atomic theory not only as a working 

 hypothesis of great value but as affording a correct description of 

 one stage of the subdivision of matter. Wliile this is undoubtedly 

 the case to-day, it is of interest to recall that less than 20 years ago 

 there was a revolt by a limited number of scientific men against 

 the domination of the atomic theory in chemistry. The followers 

 of this school considered that the atomic theory should be re- 

 garded as a mere hj^pothesis, which was of necessity un verifiable by 

 direct experiment, and should, therefore, not be employed as a 

 basis of explanation of chemistry. This point of view was much 

 strengthened by the recognition of the power of thermodynamics 

 in affording a quantitative explanation of the changes of energy in 

 chemical reactions without the assumption of any definite theory 

 of the constitution of matter. This tendency advanced so far that 

 textbooks of chemistry w^ere w^ritten in which the word atom or 

 molecule was taboo, and chemistry was based instead on the law 

 of combination in multiple proportion. At that time it did un- 

 doubtedly appear that there was little if any hope of finding a 

 concrete proof of the validity of the atomic hypothesis or of detect- 

 ing by its effects a single atom of matter or a single electron, for 

 it was known that the smallest fragment of matter visible under a 

 high-power microscope must still contain many millions, or even 

 billions, of atoms. 



The march of science has, however, been so rapid in this direction 

 that We have been able in recent years to show in a definite and 

 concrete way the independent existence of atoms and also of elec- 

 trons in rapid motion. 



