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 particular , low doses reduced the latency to and prolonged the duration of barbiturate narcosis .
### Output Text:
In particular , low doses reduced the latency to and prolonged the duration of <span class="chemical">barbiturate</span> narcosis . 