PARAMAGNETIC & DIAMAGNETIC SUBSTANCES 297 



where A is (riven by the measurements made. Pascal took /c t as 

 7'5 x 10~ 7 and KO for air as 0*25 X 10~ 7 . When coal gas or a 

 vacuum was above the liquid K O was taken as zero. 

 For a solution under air it follows that 



10 7 /c = 0-25 - 7-75X 



while for one under coal gas or a vacuum 



10 7 AC = - 7'5A. 



If the density of the solution was p and it contained/? grammes 

 of salt per c.c. there was p p of water per c.c. Assuming that salt 

 and solvent each produced its own effect, and taking x as the 

 susceptibility of the salt as dissolved, 



-7 



K p = px + (p p)7'5 x 10 



The measurement of X, /?, and p therefore gave x. For the modifi- 

 cations of the method for liquefied gases the original papers should 

 be referred to. 



From his results with solutions of salts of iron, nickel, and 

 cobalt Pascal inferred that as an ion of a ferromagnetic metal 

 |>a--r> into a complex ion or into a colloid its susceptibility 

 diminishes, and it may even become diamagnetic. Hence the 

 magnetic moment of a molecule in a given field is not the sum of 

 the moments of its constituent atoms, but depends on the grouping 

 in the molecule. 



With the diamagnetic metalloids he found that the atomic 

 Mi-ceptibihty KO. (a being the atomic weight) was nearly pro- 

 portional to e a +P a where a and /3 are constants for the same 

 family. With several diamagnetic compound gases, when liquefied 

 he found that Km (m being the molecular weight) was nearly 

 proportional to 5 -f- n, where n is the number of atoms in the 

 molecule. In diamagnetic organic compounds his results showed 

 that the different atoms preserved, as a rule, each its own 

 diAmagnetic susceptibility , but the presence of oxygen appeared to 

 lead to anomalies. 



The electron theory. The following is a brief sketch 

 of Langevin's form of the Electron Theory of Magnetism.* 



Let us, as a preliminary, imagine an atom which consists of a 

 positive charge on a large central mass, and a negative electron 

 circling round it as the moon circles round the earth. The 

 positive charge will hardly move, and the electric lines of force will 

 sweep through the space round it, all the lines directed outwards 

 from the centre, and their motion will produce a magnetic field 

 equivalent to that of a current along the orbit but opposite 

 in direction to the motion of the electron. The magnetic moment 



* Ann. tl <'!,;,. ,i ,1, Phy*. t s scr. V. (1905), p. 70. 



