716 



SCIENCE 



[Vol. LVI, No. 1460 



newly founded Kilgour eliaiir of geology at 

 Alberdeen. 



Dr. Fritz Straus, of Berlin, has been ap- 

 pointed professor of chemistry at the Breslau 

 Sehool of Teelinology. 



DISCUSSION AND CORRESPOND- 

 ENCE 

 RADIATION A FORM OF MATTER 



To THE Editor of Science: One sees the 

 statement frequently made that, if one accepts 

 Einstein's conclusion that the mass of a body 

 is proportional to the total energy which it 

 possesses, the principle of the conservation of 

 matter must be abandoned. Tor if during any 

 change energy is gained or lost by the body 

 through radiation, there should be a corre- 

 sponding gain or loss of mass. It has been 

 calculated that in the case of radioactive dis- 

 integration the energy thus lost (or gained) 

 through radiation represents an appreciable 

 fraction of the total mass of the radioactive 

 material. If, however, one takes the point of 

 view that radiation is a form of matter, and 

 that the amount of this matter is measured by 

 the mass or inertia of the radiation, the total 

 mass of the body plus that of the radiation 

 emitted is unaltered by such changes. On this 

 view the principle of the conservation of mass 

 is strictly valid, being, as has been remarked, 

 a corollary of the energy principle. 



It is perhaps surprising to notice that ac- 

 cording to the definitions of matter usually 

 given electromagnetic radiation must be classed 

 as matter. It is admittedly difficult to find a 

 satisfactory definition. "Matter is that which 

 occupies space," "matter is that which pos- 

 sesses mass or inertia," "matter is that which 

 affects the senses," are, however, common state- 

 ments. But radiation certainly occupies space; 

 that it possesses mass is shown by the mo- 

 mentum which it imparts to a body which it 

 strikes, producing radiation pressure; and who 

 would deny that sunlight affects the senses? 

 Unless, therefore, we change our idea of what 

 is meant by the word "matter," this word 

 includes not only solids, liquids and gases, but 

 also the less tangible electromagnetic radiation. 



The inclusion of radiation as a form of 



matter has important bearings in addition to 

 the fact that it renews the validity of the prin- 

 ciple of the conservation of matter. Thus, for 

 example, we can no longer say that matter is 

 composed wholly of positive and negative elee- . 

 trons, for the form of matter known as radia- 

 tion includes no such electric charges. The 

 statement that matter is composed of positive 

 and negative electrons and electromagnetic 

 radiation is, on the other hand, more complex 

 than is required. We see rather that the fun- 

 damental thing in matter is not the electric 

 charge but the electromagnetic field, for the 

 electromagnetic field includes both the elec- 

 trons and the radiation. 



If the further simplification is made of con- 

 sidering the magnetic field as due to the elec- 

 tric field in motion, we may describe all forms 

 of matter in terms of the intensity of the elec- 

 tive field at different points. The mass or 

 inertia of the matter is proportional to the 

 integral through the volume considered of the 

 square of the electric intensity and of the mag- 

 netic intensity resulting from the -motion of 

 the electric field, whether this electric field is 

 due to the presence of electrons or to the ex- 

 istence of electromagnetic radiation. The elec- 

 tric charge in an element of volume is propor- 

 tional to the divergence of the electric intensity 

 at the point. Thus all the fundamental prop- 

 erties of matter are determined if the intensity 

 of the electric field throughout space and time 

 is known. While the electrons can not be con- 

 sidered the fundamental elements which make 

 up all matter, we have thus the intensity of the 

 electric field as that which can be thought of 

 as composing both the electrons and the radia- 

 tion. Electric intensity, then, may be consid- 

 ered as that of which all matter is composed. 

 According to this point of view, matter is 

 perfectly continuous. It is true that there ai'e 

 certain perhaps limited regions, the electrons, 

 from which electric intensity diverges; but 

 whether or not these regions of divergence are 

 limited, the mass of the matter is associated 

 with the electric intensity and is hence dis- 

 tributed through all space. Similarly, radia- 

 tion propagated through space, as for example 

 light coming from the sun to the earth, is on 

 this view a continuous series of waves of 



