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: CONCLUSIONS : Both timolol solution and timolol gellan decrease the mean 24 - hour heart rate compared with placebo .
### Output Text:
CONCLUSIONS : Both <span class="chemical">timolol</span> solution and <span class="chemical">timolol</span> gellan decrease the mean 24 - hour heart rate compared with placebo . 