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 mean time of study was 109 days ( median 107 , range 4 - 184 days ) .
### Output Text:
The mean time of study was 109 days ( median 107 , range 4 - 184 days ) . 