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: In a double blind cross - over study with control group , the patients under timolol treatment presented higher depression values measured through the Beck and the Zung - Conde scales ( p < 0 . 001 vs control ) .
### Output Text:
In a double blind cross - over study with control group , the patients under <span class="chemical">timolol</span> treatment presented higher depression values measured through the Beck and the Zung - Conde scales ( p < 0 . 001 vs control ) . 