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: Passage of mannitol into the brain around gliomas : a potential cause of rebound phenomenon .
### Output Text:
Passage of <span class="chemical">mannitol</span> into the brain around gliomas : a potential cause of rebound phenomenon . 