THE PROPERTIES OF METALLIC SUBSTANCES 407 



is in a direction opposite to that observed in the case of P particles, assum- 

 ing that the conducting particles in metals are negatively charged. Since 

 a great many facts indicate that the current in metallic conductors is not 

 carried by positive particles, it appears that the various galvanomagnetic 

 effects cannot be accounted for by a simple theory of this type. A num- 

 ber of theories, that of J. J. Thomson for example, have been suggested 

 to account for the Hall effect and similar phenomena. 81 At the present 

 time, however, a satisfactory theory of these effects does not exist. In- 

 deed, the same may be said of the theory of the conduction process in 

 metals under normal conditions. It may be expected, however, that 

 ultimately the thermomagnetic and galvanomagnetic effects will play an 

 important role in the development of the theory of metallic conduction. 



A detailed study of the properties of conductors in a magnetic field 

 would lead far beyond the scope of the present monograph. The ob- 

 served facts will be found summarized in the references already given. 



11. Optical Properties of Metals. According to the electromagnetic 

 theory, the electrical and optical properties of metallic substances are 

 intimately related. The reflecting power and absorbing power of metals, 

 according to this theory, should be very great. From the known values 

 of the conductance of metallic substances, the optical constants of these 

 substances may be derived for long wave lengths when selective action 

 does not occur. 



The theory of the optical effects in metals, together with the most 

 important facts, will be found summarized in treatises on electricity and 

 magnetism and on physical optics. 32 



12. Theories Relating to Metallic Conduction. The theory of metal- 

 lic conduction, like the theory of electrolytic conduction, is still in a very 

 unsatisfactory state. Qualitatively, the theory that the current is carried 

 by negative electrons is in good agreement with the facts, but a satisfac- 

 tory quantitative theory has not, as yet, been established. The difficulties 

 confronting a comprehensive theory of metallic conduction are, indeed, 

 very great, as is apparent when it is considered how many detailed facts 

 must be accounted for. A number of theories which have been proposed 

 are able to account for a limited number of the properties of metals in a 

 fairly satisfactory manner. So, for example, the theories of Drude and 

 of Thomson render an account of the relation between the thermal and 

 the electrical conductance of metals and, to some extent, also, of the 

 thermo- and galvanomagnetic effects and thermoelectric effects in metals. 

 On the whole, however, these theories are far from satisfactory. They 



" J. J. Thomson, Rapp. Congr. Phys. 3, 143, Paris (1900). 



" See, for example, VVinkelmaun, Handbuch d. Physik ; Oraetz Handbuch d 

 trizitat u. d. Magnetismus, etc. 



