To tell the story of the development of men’s ideas regarding the nature of atmospheric air is in great part to write a history of chemistry and physics. This history is an attractive and varied one: in its early stages it was expressed in the quaint terms of ancient mythology, while in its later developments it illustrates the advantage of careful experimental inquiry. The human mind is apt to reason from insufficient premisses; and we meet with many instances of incorrect conclusions, based upon experiment, it is true, but upon experiment inadequate to support their burden. Further research has often proved the reasoning of the Schoolmen to be futile; not indeed from want of logical method, but because important premisses had been overlooked. Among the errors which misled the older speculators, three stand out conspicuously. These are-- First, _The confusion of one gas with another_. Since gases are for the most part colourless, and always transparent, they make less impression on the senses than liquids or solids do. It was difficult to believe in the substantiality of bodies which could not be seen, but the existence of which had to be inferred from the testimony of other senses; indeed, in certain instances only by the sense of touch, for many gases possess neither smell nor taste. This peculiarity led, in past ages, to the notion that air possessed a semi-spiritual nature; that its substantiality was less than that of other objects more accessible to our senses. We meet with a relic of this view in words still in common use. Thus the Greek words =πνέω=, I blow, and =πνεῦμα=, a spirit or ghost, are closely connected; in Latin we have _spiro_, I breathe, and _spiritus_, the human spirit; in English, the words _ghost_ and _gust_ are cognate. And the same connection can be traced in similar words in many other languages.