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: This experiment is discussed with regard to the hypothesis that ultrasound production is the acoustic by - product of a physiological maneuver that compensates for clonidine ' s detrimental effects on cardiovascular function .
### Output Text:
This experiment is discussed with regard to the hypothesis that ultrasound production is the acoustic by - product of a physiological maneuver that compensates for <span class="chemical">clonidine</span> ' s detrimental effects on cardiovascular function . 