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 mean differences in the pre - and post - dilatation measurements were tested for statistical significance at the 95 % level using one - tail paired t - tests .
### Output Text:
The mean differences in the pre - and post - dilatation measurements were tested for statistical significance at the 95 % level using one - tail paired t - tests . 