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: Previous studies have demonstrated that Rg1 might be a useful agent for the prevention and treatment of the adverse effects of morphine .
### Output Text:
Previous studies have demonstrated that <span class="chemical">Rg1</span> might be a useful agent for the prevention and treatment of the adverse effects of <span class="chemical">morphine</span> . 