Litmus paper, 1934, Merck Corporation
One of the simplest experiments student chemists can perform is to measure acidity levels. Litmus paper provides a qualitative means of measuring acidity by using various plant juices as color indicators. If the substance is acidic, the paper turns red.
Photo by Gregory Tobias.
From the collections at the Chemical Heritage Foundation. CHF is a non-profit library, museum, and center for scholars that's dedicated to the history of chemistry. To learn more, visit www.chemheritage.org.