As Gronon shows, the processes that made Chicago's trading in grain futures possible were based on the disentanglement of grain from its grower that took place when transport in railroad cars and storage in steam-powered grain elevators replaced transport and storage in sacks. Sacks kept grain and grower tied together, the sacks remaining the latter's property, identified as such by a bill of lading in each sack, until they reached the final purchaser. In contrast, grain from different growers was mixed irreversibly in the elevators' giant bins, and the trace of ownership was now a paper receipt, redeemable for an equivalent quantity of similar grain but not for the original physical substance.

Donald MacKenzie, An Engine, Not a Camera, loc. 71