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: Nine days later locomotor activity was measured in response to a single low dose of amphetamine ( 0 . 5 mg / kg ) .
### Output Text:
Nine days later locomotor activity was measured in response to a single low dose of <span class="chemical">amphetamine</span> ( 0 . 5 mg / kg ) . 