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: METHODS : To evaluate the potential mechanisms of AF promotion by Ca ( 2 + ) channel blockers , we administered verapamil to morphine - chloralose anesthetized dogs .
### Output Text:
METHODS : To evaluate the potential mechanisms of AF promotion by <span class="chemical">Ca</span> ( 2 + ) channel blockers , we administered <span class="chemical">verapamil</span> to <span class="chemical">morphine</span> - <span class="chemical">chloralose</span> anesthetized dogs . 