CHAP. XI.] KING'S COLLEGE, LONDON. 315 



mutual interference, except when they come into 

 collision. Maxwell showed that the apparent vis- 

 cosity of gases, their low conductivity for heat, and 

 Graham's laws of diffusion, could be satisfactorily 

 explained by this theory, and gave reasons for be- 

 lieving that in air at ordinary temperature each particle 

 experiences on an average more than 8,000,000,000 

 collisions per second. It is probable that the con- 

 templation of the "flight of brick-bats" (his own 

 vivid phrase for the constitution of Saturn's rings) led 

 him on to his far-reaching investigations in this field 

 of molecular physics. 



On the 17th of May 1861 he delivered his first 

 lecture before the Eoyal Institution. The subject was 

 " On the Theory of the three Primary Colours." 



All this while Maxwell was quietly and securely lay- 

 ing the foundations, deep and wide, of his great work on 

 Electricity and Magnetism, but he had not the leisure 

 that was requisite for bringing it to completion. 



The period of his King's College Professorship 

 was far, however, from being scientifically unfruitful. 

 The colour-box was perfected, and many series of 

 observations were made with it. Mrs. Maxwell's 

 observations were found to have a special value. 

 Through a striking discrepancy between her readings 

 and C. H. Cay's, Maxwell discovered that the blind- 

 ness of the Foramen Centrale to blue light, which 

 was strongly marked in his own dark eyes, was either 

 altogether absent from hers, or present in a very low 

 degree. The comparison of J. C. M.'s (J/s) eyes, and 

 Mrs. M.'s (K/s) is referred to in Part II. 



