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: Both timolol and pilocarpine were subsequently identified in a 24 - h collection of urine .
### Output Text:
Both <span class="chemical">timolol</span> and <span class="chemical">pilocarpine</span> were subsequently identified in a 24 - h collection of urine . 