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: This reaction demonstrates that nifedipine can seriously potentiate the toxicity of magnesium .
### Output Text:
This reaction demonstrates that <span class="chemical">nifedipine</span> can seriously potentiate the toxicity of <span class="chemical">magnesium</span> . 