We come now to that all-important transformer of power, the steam engine. Everybody knows that steam is a state of water in which, under the influence of heat, the molecules have broken away from the mutual attraction of cohesion, and are flying about at inconceivable speed, rebounding from one another after collision, in virtue of their elasticity, exerting in the aggregate an enormous pressure in every direction. It is this consideration of the intimate character of steam that justifies the title of the present chapter; a title that has further utility as drawing a contrast between the manner of working with which we are now to be concerned, and the various types of workers that we have previously considered. In speaking of the animal machine and of work accomplished by the air and the water, we have been concerned primarily with masses of matter, possessing and transmitting energy. Of course molecules--since they make up the substance of all matter--could not be altogether ignored, but in the main we have had to do with molar rather than with molecular motion. Now, however, we are concerned with a mechanism in which the molecular activities are directly concerned in performing work. Even in the aggregate the molecules make up a mere intangible gas, which requires to be closely confined in order that its energy may be made available. Once the molecules have performed their work, they are so changed in their activities that they sink back, as it were, exhausted, into a relatively quiescent state, which enables their latent cohesive forces to reduce them again to the state of a liquid. In a word, we are concerned with the manifestation of energy which depends upon molecular activities in a way quite different from what has been the case with any of the previously considered mechanisms. The tangible manifestation of energy which we term heat is not merely a condition of action and a by-product, as it was in the case of the animal machine; it is the essential factor upon which all the efficiency of the mechanism depends. It should perhaps be stated that this explanation of the action of the steam engine is a comparatively modern scientific interpretation. The earlier experimenters brought the steam engine to a high state of efficiency, without having any such conception as this of the nature of steam itself. For practical purposes it suffices to note that water when heated takes the form of steam; that this steam has the property of powerful and indefinite expansion; and thirdly, that when allowed to escape from a state of pressure, sudden expansion of the steam cools it sufficiently to cause the recondensation of part of its substance, thus creating a vacuum.