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: Nicotine ( 1 . 0 mg / kg ) caused a significant increase in locomotor activity in rats that were habituated to the test environment , but had only a weak and delayed stimulant action in rats that were unfamiliar with the test environment .
### Output Text:
<span class="chemical">Nicotine</span> ( 1 . 0 mg / kg ) caused a significant increase in locomotor activity in rats that were habituated to the test environment , but had only a weak and delayed stimulant action in rats that were unfamiliar with the test environment . 