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: While the elevations of the upper lid margin in most subjects were not more than 2 mm and did not cause noticeable change in appearance , one subject suffered from mechanical entropion and marked corneal abrasion 3 hours after instillation of the medication .
### Output Text:
While the elevations of the upper lid margin in most subjects were not more than 2 mm and did not cause noticeable change in appearance , one subject suffered from mechanical entropion and marked corneal abrasion 3 hours after instillation of the medication . 