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: The mean age of the patients was 35 . 2 years and the mean duration of therapy was 59 . 7 months .
### Output Text:
The mean age of the patients was 35 . 2 years and the mean duration of therapy was 59 . 7 months . 