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: During the acute period of status epilepticus , we measured : ( 1 ) the latency of the first motor sign , ( 2 ) the intensity of seizures , ( 3 ) the time when it occurred within the 6 - h observation period , and ( 4 ) the time when the acute period ended .
### Output Text:
During the acute period of status epilepticus , we measured : ( 1 ) the latency of the first motor sign , ( 2 ) the intensity of seizures , ( 3 ) the time when it occurred within the 6 - h observation period , and ( 4 ) the time when the acute period ended . 