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: METHODS : Forty - three Caucasian patients with primary open - angle glaucoma or ocular hypertension with a mean ( + / - SD ) age of 63 ( + / - 8 ) years were randomized and crossed over in a double - masked manner to 14 days of treatment with placebo ( morning and evening in both eyes ) , timolol solution ( morning and evening in both eyes ) , or timolol gellan ( morning in both eyes with placebo in the evening ) .
### Output Text:
METHODS : Forty - three Caucasian patients with primary open - angle glaucoma or ocular hypertension with a mean ( + / - SD ) age of 63 ( + / - 8 ) years were randomized and crossed over in a double - masked manner to 14 days of treatment with placebo ( morning and evening in both eyes ) , <span class="chemical">timolol</span> solution ( morning and evening in both eyes ) , or <span class="chemical">timolol</span> gellan ( morning in both eyes with placebo in the evening ) . 