It has been found by experiment that lights of the same intensity from different substances take different periods to vitiate the same quantity of air, by converting it into carbonic acid. Rape oil, 71 minutes j olive oil, 72 j Russian tallow, 75 j common tallow, 76 j sperm oil, 76 j stearic acid, 77 j wax candles, 79j spermaceti candles, 83j common coal gas, 98 j cannel coal gas, 152. Thus the can-nel coal gas is proved to be the most healthy to burn.