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: The aim of this study was to investigate the effect of Rg1 on learning impairment by chronic morphine administration and the mechanism responsible for this effect .
### Output Text:
The aim of this study was to investigate the effect of <span class="chemical">Rg1</span> on learning impairment by chronic <span class="chemical">morphine</span> administration and the mechanism responsible for this effect . 