An ingenious device for testing the purity of the air in workshops and other crowded places is on exhibition at Zurich. A closed vessel tilled with a chemical solution, sensitive to carbonic acid gas, sends out a drop through a glass siphon every two minutes; the drop soaks through a cord hanging vertically from the end of the siphon. If the air is very bad the drop changes its natural color, red, to white at once, at the upper end of the cord. It keeps its color along the cord in proportion to the purity of the air, not changing at all if the air is perfectly pure. A graduated scale fixed to the cord marks the degree of impurity.

From—The San Francisco Call. (San Francisco [Calif.]), 09 Aug. 1896. Chronicling America: Historic American Newspapers. Lib. of Congress.