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: Thus , the dual action of melatonin on pharmacological narcosis seems to be specific for the barbiturate mechanism of action .
### Output Text:
Thus , the dual action of <span class="chemical">melatonin</span> on pharmacological narcosis seems to be specific for the <span class="chemical">barbiturate</span> mechanism of action . 