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: Postural tremor showed no significant difference between the first and third session ( P = 0 . 07 ) .
### Output Text:
Postural tremor showed no significant difference between the first and third session ( P = 0 . 07 ) . 