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: i . p . and of 20 mg . by the intravesical route , and acrolein at doses of 0 . 5 mg . by the intravesical route and of 5 mg . / kg .
### Output Text:
i . p . and of 20 mg . by the intravesical route , and <span class="chemical">acrolein</span> at doses of 0 . 5 mg . by the intravesical route and of 5 mg . / kg . 