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: Comparison of the effect of oxitropium bromide and of slow - release theophylline on nocturnal asthma .
### Output Text:
Comparison of the effect of <span class="chemical">oxitropium bromide</span> and of slow - release <span class="chemical">theophylline</span> on nocturnal asthma . 