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 rat size to begin phenobarbitone was determined as 100 g , and this size as a group had a mean maximum relative liver weight increase 47 % greater than normal rats of the same body weight .
### Output Text:
The optimal rat size to begin <span class="chemical">phenobarbitone</span> was determined as 100 g , and this size as a group had a mean maximum relative liver weight increase 47 % greater than normal rats of the same body weight . 