282 THE CHEMISTRY OF CREATION. 



vapour is never absent from the air. When, 

 owing to some reduction of temperature, this 

 vapour passes its point of greatest density, it 

 becomes visible to the eye in the form of a 

 mist, or fog. The fogs of London have long 

 acquired, owing to their density, a proverbial 

 celebrity. " There happened," writes the amus- 

 ing John Evelyn, " this weeke, so thicke a mist 

 and fog, that people lost their waye in the 

 streetes, it being so intense, that no light of 

 candles, or torches, yielded any, or but very 

 little direction. It began about four in the 

 afternoon, and was quite gone by eight, with- 

 out any winde to disperse it." On the twenty- 

 fourth of February, 1832, an intensely thick 

 fog prevailed in the metropolis, and was so 

 thick at mid-day, that it was impossible to dis- 

 cern objects or persons distinctly; and at night, 

 the streets being illuminated in consequence of 

 some public rejoicings, boys went about the 

 streets with torches, looking, as they said, for the 

 illumination! On these two occasions, the fog 

 appears to have attained an unusual degree of 

 opacity. 



The cause of fog is considered to be the 

 intermingling of a cold and a warm current of 

 air, each pretty fully charged with watery 

 vapour: upon both thus mingling together, so 

 great a reduction in the temperature of the 



