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: From present knowledge about the effects of adrenergic eye drops on ocular blood circulation , it is difficult to suggest an association between the two events , which may be coincidental only .
### Output Text:
From present knowledge about the effects of adrenergic eye drops on ocular blood circulation , it is difficult to suggest an association between the two events , which may be coincidental only . 