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: Although paralysis after magnesium administration has been described in patients with known myasthenia gravis , it has not previously been reported to be the initial or only manifestation of the disease .
### Output Text:
Although paralysis after <span class="chemical">magnesium</span> administration has been described in patients with known myasthenia gravis , it has not previously been reported to be the initial or only manifestation of the disease . 