In addition to separating oxygen from air our vital organs are every moment performing chemical tasks just as elusive. The liver, for instance, is a sugar-maker. The elaboration of living tissue is of transcendent interest to the physiologist; it is fraught with the same attraction to the chemist who would build compounds from their elements, to the engineer who would transform heat or chemical energy into motive power with less than the enormous loss of our present methods. Flight. In 1887 the late Professor S. P. Langley of Washington began experiments in mechanical flight. He found that one horse-power will support in calm air and propel at forty-five miles an hour a wing-plane weighing 209 pounds. Dr. A. F. Zahm, of the Catholic University of America, at Washington, has recently ascertained that a thin foot-square gliding plane weighing one pound soars with the least expenditure of power at about 40 miles an hour, while at 80 miles the power required is more than twice as much. As engines have been made weighing less than ten pounds per horse-power, capable of yielding a horse-power for five hours with four pounds of oil, we are plainly approaching the mastery of the air,--so freely exercised by the sparrow and the midge. Among the students eager in this advance are the men who examine with the camera how wings of diverse types behave in flight, and then endeavor to imitate the strongest and swiftest of these wings. Light. Professor Langley conducted another inquiry of fascinating interest, this time respecting those natural light-producers, the fireflies, especially the large and brilliant species indigenous to Cuba, _Pyrophorus noctilucus_. As the result of refined measurements with the spectroscope and the bolometer, the most delicate heat detector known to the laboratory, he said: “The insect spectrum is lacking in rays of red luminosity and presumably in the infra-red rays, usually of relatively great heat, so that it seems probable that we have here light without heat.” When we remember that ordinary artificial light is usually accompanied by fifty to a hundred times as much energy in the form of wasteful and injurious heat, we see the importance of this research. If light can be produced without heat by nature, why not also by art? [Illustration: Cuban firefly, life size.] Converting Heat Into Work.