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: During the night , the mean 12 - hour heart rate on placebo and timolol gellan were both significantly less than on timolol solution ; the difference between solution and gellan treatments was statistically significant ( P = . 01 ) .
### Output Text:
During the night , the mean 12 - hour heart rate on placebo and <span class="chemical">timolol</span> gellan were both significantly less than on <span class="chemical">timolol</span> solution ; the difference between solution and gellan treatments was statistically significant ( P = . 01 ) . 