About: http://webisa.webdatacommons.org/250917334

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	During the 19th and early 20th centuries, the skies above such major cities as Chicago and Pittsburgh were dark with smoke from steel smelters, heavy industrial plants, and coal burning.
http://www.w3.org/ns/prov#wasQuotedFrom	blogspot.com