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: This paper presents the results of treating IST by intratympanic instillation of lignocaine ( lidocaine ) 2 per cent through a grommet , for five weekly courses .
### Output Text:
This paper presents the results of treating IST by intratympanic instillation of <span class="chemical">lignocaine</span> ( <span class="chemical">lidocaine</span> ) 2 per cent through a grommet , for five weekly courses . 