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 Multiple Sleep Latency Test was performed at 11 : 30 a . m . , 2 : 00 p . m . , and 4 : 30 p . m . on days 2 , 8 , and 15 .
### Output Text:
The Multiple Sleep Latency Test was performed at 11 : 30 a . m . , 2 : 00 p . m . , and 4 : 30 p . m . on days 2 , 8 , and 15 . 