Oneiry, in sixth grade and 11 years old, liked the tie-dye experiment, where learning about the light and color also resulted in cool take-home T-shirts. Genesis, a nine-year-old fourth grader, really enjoyed the liquid nitrogen demonstration, especially the ice cream she got to make with it. And Julia, at 10 in fifth grade, had a good time making "gak," a substance that's not quite solid and not quite liquid - and slimy and fun. They were among 10 Middletown girls between fourth and sixth grade who participated in a girls' science camp sponsored by the Green Street Arts Center Aug. 4-8. The session,…