MODERN IDEAS ON THE CONSTITUTION OF MATTER. 1 



By Jean Becqtjerel, 

 Professor at the National Museum of Natural History, Paris. 



For a number of years past physicists have been laying the 

 foundations of a new theory of matter. A series of bold concep- 

 tions, based on unlooked-for facts, has worked a deep-seated trans- 

 formation in the previously accepted ideas concerning the constitu- 

 tion of bodies. 



Everyone knows that substances in general are divided into two 

 groups, simple bodies or elements and complex bodies made up by 

 the combination of these elements. For a long time these bodies 

 have been considered as composed of atoms which have combined 

 and formed molecules, the atom being the most minute quantity of 

 matter characteristic of an element and capable of entering into 

 chemical combinations, while the molecule of a body, simple or com- 

 plex, is the smallest particle of this body which is capable of exist- 

 ing in a physical state. Let us consider an example: The molecule 

 of water, the smallest quantity of water which can exist in a physical 

 state, is the result of the combination of two atoms of hydrogen 

 with one of oxygen. I shall repeat before you the classic experiment 

 of decomposing water by an electric current; oxygen is set free at 

 the positive pole and hydrogen at the negative pole, the two gases 

 coming off in the proportion of two volumes of hydrogen to one 

 volume of oxygen. 



The molecule of a complex body is always made up of the atoms 

 of at least two elements. The molecule of an element may be made 

 up of only a single atom, as is the case with monatomic bodies such 

 as helium, zinc, cadmium, or mercury, while in other cases the 

 molecule of a simple body may be a group of several atoms of this 

 body, for instance, hydrogen and oxygen are diatomic, while phos- 

 phorus and arsenic are tetratomic. 



The discovery of Gay Lussac concerning the laws of the compo- 

 sition of gases led Avogadro and Ampere to declare that gases con- 



1 Lecture delivered at the Museum, Apr. 10, 1910. Translated by permission from the 

 Revue Scientifique, Paris, 48, No. 14, Oct. 1, 1910. 



275 



