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: Apamin ( 10 ng ) had a tendency to decrease the convulsive threshold ( 21 . 6 + / - 2 . 2 to 19 . 9 + / - 2 . 5 mg . l ( - 1 ) ) but this was not statistically significant .
### Output Text:
<span class="chemical">Apamin</span> ( 10 ng ) had a tendency to decrease the convulsive threshold ( 21 . 6 + / - 2 . 2 to 19 . 9 + / - 2 . 5 mg . l ( - 1 ) ) but this was not statistically significant . 