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: The convulsive threshold ( mean + / - SD ) was 41 . 4 + / - 6 . 5 mg .
### Output Text:
The convulsive threshold ( mean + / - SD ) was 41 . 4 + / - 6 . 5 mg . 