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: Rc seemed to be independent of age and decreased with increasing serum concentration of cimetidine , suggesting that secretion of cimetidine is a saturable process .
### Output Text:
Rc seemed to be independent of age and decreased with increasing serum concentration of <span class="chemical">cimetidine</span> , suggesting that secretion of <span class="chemical">cimetidine</span> is a saturable process . 