In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: During the early stages of analgesic treatment , the changes in urinary concentrating ability were reversible , but after prolonged analgesic treatment , maximum urinary concentrating ability failed to recover .
### Output Text:
During the early stages of analgesic treatment , the changes in urinary concentrating ability were reversible , but after prolonged analgesic treatment , maximum urinary concentrating ability failed to recover . 