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: There was mutual potentiation between sub - effective doses of ketamine and morphine , but sub - effective doses of ketamine partly antagonized fully - effective doses of morphine .
### Output Text:
There was mutual potentiation between sub - effective doses of <span class="chemical">ketamine</span> and <span class="chemical">morphine</span> , but sub - effective doses of <span class="chemical">ketamine</span> partly antagonized fully - effective doses of <span class="chemical">morphine</span> . 