King Coal by Upton Sinclair

The mule is notoriously a profane and godless creature; a blind alley of Nature, so to speak, a mistake of which she is ashamed, and which she does not permit to reproduce itself. The thirty mules under Hal's charge had been brought up in an environment calculated to foster the worst tendencies of their natures. He soon made the discovery that the "colic" of his predecessor had been caused by a mule's hind foot in the stomach; and he realised that he must not let his mind wander for an instant, if he were to avoid this dangerous disease.

These mules lived their lives in the darkness of the earth's interior; only when they fell sick were they taken up to see the sunlight and to roll about in green pastures. There was one of them called "Dago Charlie," who had learned to chew tobacco, and to rummage in the pockets of the miners and their "buddies. " Not knowing how to spit out the juice, he would make himself ill, and then he would swear off from indulgence. But the drivers and the pit-boys knew his failing, and would tempt "Dago Charlie" until he fell from grace. Hal soon discovered this moral tragedy, and carried the pain of it in his soul as he went about his all-day drudgery.