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: Magnesium levels in patients on lithium treatment were unchanged from baseline levels .
### Output Text:
<span class="chemical">Magnesium</span> levels in patients on <span class="chemical">lithium</span> treatment were unchanged from baseline levels . 