But before explaining this theory more fully and seeing upon what experimental evidence it is based, it would be well to consider its genesis and briefly recall the ancient notions regarding 'artificial' light.
Light was first confused with seeing, and it is said that up to the time of Aristotle men commonly thought they saw by reason of something shooting out from the eyes and coming in contact with objects; the converse of the Cartesian conception of many centuries later, that certain movements in bodies cause them to shoot out minute particles in all directions, which, striking the eye or causing 'globules' of air to strike it, excite vision.
The fluid nature of fire and the corporeal nature of light, which were believed in throughout the early and middle ages, seem to have been first doubted by Sir Francis Bacon about the end of the sixteenth century, although he was by no means sure that these conceptions were wrong. Bacon classed together the light from flames, decayed wood, glowworms, silks, polished surfaces, etc., and said that inasmuch as some animals can see in the dark, air has some light of itself. Boerhaave, somewhat later, also expressed doubts as to the substantive nature of fire.
Among the first recorded experiments upon the nature and action of luminous flames are those which were carried out by Sir Robert Boyle between 1660 and 1670. He attempted to prove by experiment whether the light from a flame is like that from the sun, and whether it is corporeal or merely a quality. He allowed a flame to play on metals directly and also when in open and sealed vessels, and because the substance formed a calx and gained in weight, he thought that the light or flame (he uses the terms indiscriminately) had combined with the metal, and hence it must be a fluid. Boyle also conducted a large number of experiments upon live or 'quick' coals, phosphorescent bodies, animals and insects to see the effect of exhausting a receiver in which they were placed, and he seems to have concluded that the lights from live coals, rotten wood and putrefying fish differ not in kind but only in degree. He considered that the increase of light from coals, etc., and the reviving of certain insects when air was readmitted to the receiver indicated a relation between a visible flame and the so-called 'vital flame.' But he would not commit himself upon the question of the supposed kinship between the 'flame' from live coals and rotten wood and the 'vital flame' thought to be burning in the hearts of all living beings.
The interesting views of Sir Isaac Newton are set forth in a number of queries published in his work entitled 'Optics.' As is well known, Newton believed in the material nature of light, and he asserted that the change of light into matter and of matter into light is an acknowledged possibility and of common occurrence. He attributed the light
