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: He remained thirsty and polyuric despite cessation of lithium and investigations on admission showed him to have normal osmoregulated thirst and vasopressin secretion , with clear evidence of nephrogenic diabetes insipidus .
### Output Text:
He remained thirsty and polyuric despite cessation of <span class="chemical">lithium</span> and investigations on admission showed him to have normal osmoregulated thirst and <span class="chemical">vasopressin</span> secretion , with clear evidence of nephrogenic diabetes insipidus . 