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: Hemorrhage was measured and the visibility of the operative field was evaluated according to a six - point scale .
### Output Text:
Hemorrhage was measured and the visibility of the operative field was evaluated according to a six - point scale . 