SOME coxTEMPOR.-iRy .iDf.ixcr.s i\ pnysics rni 457 



interpreting the lines ns the results of tr.insiiiDiis hetween sialionarN- 

 states. 



!'. M\(.\i III Mii\ii;nts f)F Atoms 



Of the enormous and chaotic variety of facts about the nianneli- 

 pro[x"rties of materials, only a few of the least conspicuous have been 

 serviceable to atom-builders; the notorious ones have helped very 

 little or not at all. The famous and characteristic magnetic prop- 

 erties of iron, nickel, cobalt, depend on the arranRement of the atoms 

 and on the temperature of the metal, and cannot safely be attributed 

 to the atoms themsehes. Diamagnetism, an inconspicuous and 

 rarely-mentioned (juality of certain elements, is in some instances 

 (|uite independent of temperature, and may well be a properly of 

 the atoms. Paramagnetism, an almost ctjualh' incons(iicuous quality 

 of certain other elements, depends on temperature, but in such a way 

 that it may sometimes be explained b\- assuming that each atom has 

 a characteristic magnetic moment, the same for all the atoms of a 

 substance. The value of this magnetic moment of the atom may be 

 calculated from measurements on the paramagnetism of the sub- 

 stance; the process of calculation invokes certain assumptions, at 

 least one of which is at the present open to question. 



Direct measurements upon the magnetic moments of certain atoms 

 are now being made by C.erlach; and they are among the most im- 

 portant achievements of these years. In a small electric oven, a 

 metal such as silver is vaporized; a beam of the outflowing atoms, 

 passing through a small orifice in the wall of the oven and through 

 others beyond this one, eventually tra\cls across a strong magnetic 

 field with a strong field-gradient and falls upon a plate. Suppose 

 that each atom is a bar-magnet, oriented with its length parallel to 

 the magnetic field. If the field were uniform, the bar-magnet would 

 not be deflected, it would travel across the field in a straight line; 

 for although its north pole would be drawn sidewise by a force, its 

 south pole would be pushed by an exactly equal force in the exactly 

 oppf)site direction. That the atom may be drawn aside, the field 

 must be perceptibly different at two points as close together as the 

 two poles of the magnet. When one considers how small an object 

 the atom is, it is clear that the field must change very rapidly from 

 one fx)int of space to another, its gradient must be enormous. Gerlach 

 succeeded in contriving so great a magnetic field with so great a 

 gradient that the beam of flying atoms was perceptibly drawn aside. 

 The most-deflected atoms are those of which the magnetic axes are 

 most nearly parallel to the magnetic field. From their deflections, 



