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: Empirical treatment with intravenous calcium gluconate was initiated , and muscle contractions slowly subsided over approximately 10 to 15 minutes .
### Output Text:
Empirical treatment with intravenous <span class="chemical">calcium gluconate</span> was initiated , and muscle contractions slowly subsided over approximately 10 to 15 minutes . 