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: Improvements were seen in the number of tender joints , the severity of swelling and tenderness , the time of walk 50 feet , the duration of morning stiffness and the circumference of the left knee .
### Output Text:
Improvements were seen in the number of tender joints , the severity of swelling and tenderness , the time of walk 50 feet , the duration of morning stiffness and the circumference of the left knee . 