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: Basal activity , latent period and the penicillin - induced epileptiform activity periods were then analyzed using both conventional methods and spectral analysis .
### Output Text:
Basal activity , latent period and the <span class="chemical">penicillin</span> - induced epileptiform activity periods were then analyzed using both conventional methods and spectral analysis . 