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: The beta adrenergic blocking drug , timolol , tended to correct the hypokalemia of short - term bendrofluazide treatment in 6 healthy male subjects and although the effect was small it was significant .
### Output Text:
The beta adrenergic blocking drug , <span class="chemical">timolol</span> , tended to correct the hypokalemia of short - term <span class="chemical">bendrofluazide</span> treatment in 6 healthy male subjects and although the effect was small it was significant . 