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: Streptomycin sulfate ( 300 mg / kg s . c . ) was injected for various periods into preweanling rats and for 3 weeks into weanling rats .
### Output Text:
<span class="chemical">Streptomycin</span> sulfate ( 300 mg / kg s . c . ) was injected for various periods into preweanling rats and for 3 weeks into weanling rats . 