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: The optimal time for the initial dose of carbon tetrachloride was after 14 days on phenobarbitone .
### Output Text:
The optimal time for the initial dose of <span class="chemical">carbon tetrachloride</span> was after 14 days on <span class="chemical">phenobarbitone</span> . 