But what, then, is mind? Is it a mere phenomenon, accompanying the physico-chemical reactions of life and vanishing with the end of the reaction, just as the phenomenon of a flame may accompany a chemical reaction, and vanish when the reaction is completed? Or is mind an entity, just like the entity energy and the entity matter, but differing from either of them--in short, a third entity? We have compared mind with the phenomenon of a flame accompanying a chemical reaction; but, after all, the flame is not a mere phenomenon, but is an entity, is energy. More than once, in the apparently continuous and unbroken structure of science, wide gaps have been discovered into which new sections of knowledge fitted, sections the existence of which had never been suspected. So in Mendelejeff's _Periodic System of the Elements_ all chemical elements fitted in without gaps--in a continuous series (except a few missing links, which were gradually discovered and filled in). Nevertheless, the whole group of six noble gases, from helium to emanium, were discovered and fitted into the periodic system at a place where nobody had suspected a gap. One of the most interesting of such unsuspected gaps in the structure of science is the following, because of its pertinency to the subject of our discussion. In studying the transformations of matter, the chemist records them by equations of the form: (1) 2H_{2} + O_{2} = 2H_{2}O, which means: Two gram molecules of hydrogen H_{2}(2 X 2 = 4 grams) and 1 gram molecule of oxygen O_{2}(1 X 32 grams), combine to 2 gram molecules of water vapor H_{2}O (2 X 18 = 36 grams). For nearly a hundred years chemists wrote and accepted this equation; innumerable times it has been experimentally proved by combining 4 parts of hydrogen and 32 parts of oxygen to 36 parts of water vapor; so that this chemical equation would appear as correct and unquestionable as anything can be. Nevertheless, it is wrong, or rather incomplete. It does not give the whole event, but omits an essential part of it, and now we write it: (2) 2H_{2} + O_{2} = 2H_{2}O + 293,000 J., which means: The matter _and energy_ of 2 gram molecules of hydrogen, and the matter _and energy_ of 1 gram molecule of oxygen, combine to the matter _and energy_ of 2 gram molecules of water vapor and 293,000 joules, or units, of _free energy_.