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: Comparison of aqueous and gellan ophthalmic timolol with placebo on the 24 - hour heart rate response in patients on treatment for glaucoma .
### Output Text:
Comparison of aqueous and gellan ophthalmic <span class="chemical">timolol</span> with placebo on the 24 - hour heart rate response in patients on treatment for glaucoma . 