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: Nineteen patients finished the trial , and in 18 cases the therapeutic result was considered very good to good .
### Output Text:
Nineteen patients finished the trial , and in 18 cases the therapeutic result was considered very good to good . 