Every-day Science: Volume 6. The Conquest of Nature

by Henry Smith Williams; Edward Huntington Williams

CAPTIVE MOLECULES: THE STORY OF THE STEAM ENGINE

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.

Stated in few words, the entire action of the steam depends upon these simple mechanical principles. The principles are practically applied by permitting the steam to enter the cylinder where it can act on a piston, to which it gives the thrust that is transmitted to an external mechanism by means of a rod attached to the piston. When the piston has been driven to the end of the desired thrust, the valve is opened automatically, permitting the steam to escape, thus producing a vacuum, and insuring the return thrust of the piston, which is further facilitated, ordinarily, by the admission of steam to the other side of the piston. Practical operation of this mechanism is familiar to everyone, though the marvel of its power and efficiency seems none the less because of its familiarity.

It is not too much to say that this relatively simple device, in its first general application, marked one of the most important turning points in the history of civilization. To its influence, more than to any other single cause, must be ascribed the revolutionary change that came over the character of practical life in the nineteenth century. From prehistoric times till well toward the close of the eighteenth century, there was scarcely any important change in carrying out the world's work. And in the few generations that have since elapsed, the entire aspect of the mechanical world has been changed, the working efficiency of the individual has been largely increased; mechanical tasks have become easy which hitherto were scarcely within the range of human capacity.

Before we go on to the detailed study of the machine which has produced these remarkable results, it is desirable to make inquiry as to the historical development of so important an invention.

The practical steam engine in its modern form dates, as just mentioned, from the latter part of the eighteenth century, and was perfected by James Watt, who is commonly thought of as being its inventor. In point of fact, however, the history of most inventions is duplicated here, as on examination it appears that various forerunners of Watt had been on the track of the steam engine, and some of them, indeed, had produced a workable machine of no small degree of efficiency.

The very earliest experiments were made away back in the Alexandrian days in the second century before the Christian era, the experimenter being the famous Hero, whose work in an allied field was referred to in the preceding chapter. Hero produced—or at least described and so is credited with producing, though the actual inventor may have been Ctesibius—a little toy mechanism, in which a hollow ball was made to revolve on an axis through the agency of steam, which escaped from two bent tubes placed on opposite sides of the ball, their orifices pointing in opposite directions. The apparatus had no practical utility, but it sufficed to establish the principle that heat, acting through the agency of steam, could be made to do mechanical work. Had not the age of Hero been a time of mental stasis, it is highly probable that the principle he had thus demonstrated would have been applied to some more practical mechanism in succeeding generations. As it was, however, nothing practical came of his experiment, and the steam turbine engine was remembered only as a scientific toy.

No other worker continued the experiments, so far as is known, until the time of the great Italian, Leonardo da Vinci, who, late in the fifteenth century, gave a new impulse to mechanical invention. Leonardo experimented with steam, and succeeded in producing what was virtually an explosion engine, by the agency of which a ball was propelled along the earth. But this experiment also failed to have practical result.