Adding acid to the test tube starts a chemical process which leads to the formation of crystals.把酸加入试管就会产生化学反应，最后形成晶体。The tube scents of sulphur.这个试管有硫磺气味。When acid is present, the chemical in the test tube turns red.存在酸的话，试管内的化学物就会变成红色。Scientists are studying these cells in vitro.科学家们把这些细胞放在试管里进行研究。