Note 1 — HOME STUDIES IN CHEMISTRY AND PHYSICS.

“Grotto del Cane,” grotˈto del caˈnā. Grotto of the dog. An excavation at the foot of a hill not far from Naples. The “American Cyclopædia” says of it: “It would seem from Pliny’s reference to it as ‘Charon’s ditches,’ that in his time the mephitic gas for which it is still remarkable was exhaled in quantity sufficient to prove fatal to human life. At the present time this forms but a shallow stratum on the floor, in which a candle is extinguished and dogs are stifled by way of experiment. The custom of exhibiting the effect of the carbonic acid gas upon dogs has given the distinctive name to the grotto.”