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: A slow bolus of subhypnotic doses of ketamine ( 0 . 25 mg / kg or 0 . 50 mg / kg ) was given to 10 cancer patients whose pain was unrelieved by morphine in a randomized , double - blind , crossover , double - dose study .
### Output Text:
A slow bolus of subhypnotic doses of <span class="chemical">ketamine</span> ( 0 . 25 mg / kg or 0 . 50 mg / kg ) was given to 10 cancer patients whose pain was unrelieved by <span class="chemical">morphine</span> in a randomized , double - blind , crossover , double - dose study . 