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: Pretreatment with ammonium acetate ( NH4Ac ) ( 6 mmol / kg s . c . ) approximately doubled the time morphine - treated mice remained on a hot surface and similarly increased muscular incoordination by diazepam , but NH4Ac treatment alone had no effect .
### Output Text:
Pretreatment with <span class="chemical">ammonium acetate</span> ( <span class="chemical">NH4Ac</span> ) ( 6 mmol / kg s . c . ) approximately doubled the time <span class="chemical">morphine</span> - treated mice remained on a hot surface and similarly increased muscular incoordination by <span class="chemical">diazepam</span> , but <span class="chemical">NH4Ac</span> treatment alone had no effect . 