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: RESULTS : Hallucinations occurred only when the patient ' s eyes were closed and , in all but one case , were reported as disturbing or frightening .
### Output Text:
RESULTS : Hallucinations occurred only when the patient ' s eyes were closed and , in all but one case , were reported as disturbing or frightening . 