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: DESIGN : This trial evaluated the effect of placebo , 0 . 5 % aqueous timolol ( timolol solution ) and a 0 . 5 % timolol suspension that forms a gel on application to the conjunctiva ( timolol gellan ) on the 24 - hour heart rate in patients currently being treated for glaucoma to quantify the reduction in mean heart rate .
### Output Text:
DESIGN : This trial evaluated the effect of placebo , 0 . 5 % aqueous <span class="chemical">timolol</span> ( <span class="chemical">timolol</span> solution ) and a 0 . 5 % <span class="chemical">timolol</span> suspension that forms a gel on application to the conjunctiva ( <span class="chemical">timolol</span> gellan ) on the 24 - hour heart rate in patients currently being treated for glaucoma to quantify the reduction in mean heart rate . 