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: We performed a preliminary study in order to determine any difference between a non selective beta - blocker ( timolol ) and a selective beta - blocker ( betaxolol ) regarding CNS side effects .
### Output Text:
We performed a preliminary study in order to determine any difference between a non selective beta - blocker ( <span class="chemical">timolol</span> ) and a selective beta - blocker ( <span class="chemical">betaxolol</span> ) regarding CNS side effects . 