SOME CONTEMPORARY ADVANCES IN PlfVSfCS—X 115 



5, p, d and other sequences. The Normal Stale of tlic ionized-calcium 

 atom belongs to the 5-sequence; following the usual custom it may 

 be called the (1, s) State. The Stale- of next lowest energy-value, 

 the "next-to-normal" State (so to speak) belongs to the (/-sequence, 

 and may be called the (3, d) State. The energy-difiference between 

 the (1, 5) State and the (3, d) State is 1.69 volts. This agrees within 

 the error of the experiments with that value 1.72 equivalent volts, 

 which was found for the energy-difiference between the two conditions, 

 in either of which the residue of the calcium atom might be left after 

 the twentieth electron is abstracted. This agreement shows that the 

 extraction of the 20th electron from a calcium atom may leave the 

 residue either in the (1, s) State or in the (3, d) State. 



If now we remember that the ionized-calcium atom is comparable 

 with the potassium atom (and with alkali-metal atoms generally) 

 having as it does eighteen electrons very tightly bound as a kernel 

 around the nucleus and one electron loosely held — then it is reason- 

 able to use the same interpretation of its Stationary States as was 

 expounded in Section P; and to suppose that when the ionized-cal- 

 cium atom is in the {1, s) State that loosely-held electron is revolving 

 in a certain Wi orbit, and when the atom is in the (3, d) State the 

 electron is revolving in a certain n^ orbit. Thus the extraction of 

 the 20th electron of the calcium atom may be supposed to leave the 

 19th electron sometimes in the one, sometimes in the other of these 

 two orbits. 



We may now inquire whether the 19th electron will ahvays remain 

 in its Hi orbit, or in its Us orbit as the case may be, when the 20th 

 electron reenters the atom, descending from one orbit to another. 

 Here it is necessary to w^atch one's mental steps very closely; for one 

 is liable to slip into the naive notion of a particular orbit, say for 

 instance a 33 orbit, as a fixed and permanent railway-track around 

 which the electron continually runs until something violent derails 

 it. This could not be true unless (to take this special case) the 2 )th 

 electron had no influence whate\-er upon the 19th. Were it so, every 

 Stationary State of the one family would differ by the same amount. 

 1.69 equivalent volts, from the corresponding State of the other 

 family. In fact, the energy-difference between corresponding States 

 varies from one pair to another. This may well be simply because 

 the approach of the 2:th electron so alters the forces acting upon 

 the 19th, that its orbit is changed both in geometr\- and in energy- 

 value, while remaining still identified with the same values of its 

 quantum-numbers. The experiments neither prove nor disprove 

 this; it is commonly accepted as true. 



