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: On the whole , the differences between midazolam and thiopentone had no apparent clinical consequences .
### Output Text:
On the whole , the differences between <span class="chemical">midazolam</span> and <span class="chemical">thiopentone</span> had no apparent clinical consequences . 