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: In 2 of the 3 cases the patients were also taking lithium carbonate and beta - blockers , both of which could have contributed to the incontinence .
### Output Text:
In 2 of the 3 cases the patients were also taking <span class="chemical">lithium carbonate</span> and beta - blockers , both of which could have contributed to the incontinence . 