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: S - A conduction through two pathways , the first with 2 / 1 block the second having 0 . 12 - 0 . 14 s longer conduction time and with occasional 2 / 1 block was proposed for the explanation of the alternating P - P interval and other electrocardiographic features seen .
### Output Text:
S - A conduction through two pathways , the first with 2 / 1 block the second having 0 . 12 - 0 . 14 s longer conduction time and with occasional 2 / 1 block was proposed for the explanation of the alternating P - P interval and other electrocardiographic features seen . 