It is this lack of combining power, of course, that explains the non-discovery of these elements during all these years, for the usual way of testing an element is to bring it in contact with other substances under conditions that permit its atoms to combine with other atoms to the formation of new substances. But in the case of new elements such experiments as this have not proved possible under any conditions as yet attained, and reliance must be had upon other physical tests--such as variation of the bulk of the gas under pressure, and under varying temperatures, and a study of the critical temperatures and pressures under which each gas becomes a liquid. The chief reliance, however, is the spectroscope--the instrument which revealed the presence of helium in the sun and the stars more than a quarter of a century before Professor Ramsay ferreted it out as a terrestrial element. Each whiff of colorless gas in its test-tube interferes with the light passing through it in such a way that when viewed through a prism it gives a spectrum of altogether unique lines, which stamp it as krypton, neon, or zenon as definitely as certain familiar and more tangible properties stamp the liquid which imprisons it as mercury. QUERIES SUGGESTED BY THE NEW GASES Suppose that a few years ago you had asked some chemist, "What are the constituents of the atmosphere?" He would have responded, with entire confidence, "Oxygen and nitrogen chiefly, with a certain amount of water-vapor and of carbonic-acid gas and a trace of ammonia." If questioned as to the chief properties of these constituents, he would have replied, with equal facility, that these are among the most important elements; that oxygen might almost be said to be the life-giving principle, inasmuch as no air-breathing creature could get along without it for many moments together; and that nitrogen is equally important to the organism, though in a different way, inasmuch as it is not taken up through the lungs. As to the water-vapor, that, of course, is a compound of oxygen and hydrogen, and no one need be told of its importance, as every one knows that water makes up the chief bulk of protoplasm; carbonic-acid gas is also a compound of oxygen, the other element this time being carbon, and it plays a quite different rôle in the economy of the living organism, inasmuch as it is produced by the breaking down of tissues, and must be constantly exhaled from the lungs to prevent the poisoning of the organism by its accumulation; while ammonia, which exists only in infinitesimal quantities in the air, is a compound of nitrogen and hydrogen, introducing, therefore, no new element.