Sentence:  (187) DRY FOG — Dense fogs have been noticed around London when the humidity was only 50 to 80 per cent. Frankland believes [...]. While this explanation is probably true, we do not know under what conditions a fog of oil and water gives drops of water coated with oil or drops of oil coated with water. [...] In view of the fact that lampblack enables us to emulsify water in oil, a smoky atmosphere might well be conducive to the production of <WSD>dry fogs</WSD>. 
Meaning of the word marked with <WSD> tags:  A fog of liquid droplets, occurring when the humidity or dew point would not normally allow fog, due to the droplets consisting of or coated in an oil that prevents them from evaporating.