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: The effect of treatment with gum Arabic on gentamicin nephrotoxicity in rats : a preliminary study .
### Output Text:
The effect of treatment with <span class="chemical">gum Arabic</span> on <span class="chemical">gentamicin</span> nephrotoxicity in rats : a preliminary study . 