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: Control rats received halothane anesthesia ( 1 MAC ) for one hour , followed by SNP infusion , 40 microgram / kg / min , for 30 min , followed by a 30 - min recovery period .
### Output Text:
Control rats received <span class="chemical">halothane</span> anesthesia ( 1 MAC ) for one hour , followed by <span class="chemical">SNP</span> infusion , 40 microgram / kg / min , for 30 min , followed by a 30 - min recovery period . 