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: However , the Rg1 and Rb1 ginsenosides failed to prevent OIH in either test .
### Output Text:
However , the <span class="chemical">Rg1 and Rb1 ginsenosides</span> failed to prevent OIH in either test . 