Intelligence and Miscellaneous Articles. 337 



intensity I m is nearly equal to three times the critical intensity I c ; 

 in fact in the preceding example I c =478 and I OT =1400, a third of 

 which is 467. Now this property of ordinary fluids may be 

 deduced from the equation of Van der Waals ; we are thus led to 

 inquire whether we may compare the magnetic properties to those 

 of fluids. 



3. Following Faraday, let us regard the field as occupied by 

 tubes of force whose section a at each point is inversely as the 

 field, 0-11 = 1. In other words, instead of representing the intensity 

 of the field by the number of lines of force per unit surface, let us 

 define it by the section of unit tube. The coefficient K may then 

 be written lo- ; this is the quantity of magnetism contained in the 

 section of unit tube, just as the temperature is the quantity of 

 heat contained in unit volume of a gas. 



Between the superficial density I and the susceptibility K there is 

 the same relation as between the cubical density d and the temperature 

 T of a fluid, a relation expressed by the formula of Van der 

 Waals. Given a fluid obeying this law, 



ET c 



v — a v' 2 



If it is gaseous and very far from its point of saturation, it 

 virtually follows the law of Mariotte, and we may write ^w=RT, 

 from which is deduced the approximate law of its expansion, 



•^ = ET. 



av 



The expansion of unit volume is proportional to the absolute 

 temperature. This expression leads us to Frohlich's law. 



Assume, in fact, that we can apply to magnetic phenomena the 



same equation, in which we shall replace a by =-, T by K, and 



v bv - ; we shall have 



EKH m p 



2- I _i c1, 



% -■- 



Admitting, in conformity with a well-known demonstration, that 

 the pressure exerted against the external medium on the surface is 

 proportional to I 2 <r, that is to say to KI, it will be seen that to 

 write the equation p = B-KT, is to express a law analogous to that 

 of Mariotte. We deduce from this, 



EF<r = ?*H??-cI 2 , 



