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: Heart weight increased 44 % after 8 days of treatment with a half time of 3 . 4 days .
### Output Text:
Heart weight increased 44 % after 8 days of treatment with a half time of 3 . 4 days . 