CHARLES AUGUSTIN COULOMB 149 



according to methods in some respects very different. It 

 implies, since the mean density of all substances on the sur- 

 face of the earth is much smaller, that heavy substances, no 

 doubt metals, must be predominant in the centre of the 

 earth. This is to be expected if the earth was originally a 

 molten mass, since the heavy substances would have col- 

 lected near its centre, but we thus know for certain from the 

 density as actually measured, that these substances must be 

 present in the interior in considerable amount. 



CHARLES AUGUSTIN COULOMB 

 ly^S-iSoG 



' AND HIS PREDECESSORS 



With Coulomb, the knowledge of the peculiar electrical 

 and magnetic phenomena, which had been dimly appre- 

 hended from early times, begins to develop into a science; 

 for he made the first quantitative determinations of the laws 

 of both phenomena; the two 'Coulomb's laws.' Upon this 

 followed a rapid and quite unexpected development; on the 

 one hand, fragmentary existing knowledge concerning 

 electrical phenomena was expanded into a complete system 

 of 'electrostatics' (theory of stationary electricity), and on 

 the other hand, magnetism was provided with the foundation 

 upon which Gauss and Weber were later able to build. 

 This then led to the quantitative mastery of all electrical 

 and magnetic quantities, which finds its expression in the 

 construction of the system of units generally used to-day, 

 in which - by way of recognition of the historical connection 

 - the technical unit of electrical quantity is called after 

 Coulomb. 



Coulomb's law of electricity states the forces with which 

 electric charges attract or repel one another, and tells us 



