free and perfectly elastic molecules in a state of motion. 
7 
found to belong to aeriform bodies. Inasmuch, therefore, as the word may be applied 
to a simple unmixed gas so as to speak of it as an oxygen medium or a hydrogen 
medium, &c., so far we may be allowed the use of it in treating of a hypothetical 
medium, which we have carefully to refrain from assimilating to any known form of 
matter until, by synthetical reasoning, circumstantial evidence has been accumulated 
sufficient to prove or render probable its identity. 
To have a proper conception of what the medium is that forms the subject of 
speculation, we must imagine a vast multitude of small particles of matter, perfectly 
alike in every respect, perfectly elastic as glass or ivory—but of size, form and 
texture that requires not to be specified further than that they are not liable to 
change by mutual action—to be enclosed by elastic walls or surfaces in a space so 
much greater than their aggregate bulk as to allow them freely to move amongst 
each other in every direction. As all consideration of attractive forces is left out at 
present, it is obvious that each particle must proceed on a straight line until it strikes 
against another, or against the sides of the enclosure ; that it must then be reflected 
and driven into another line of motion, traversing backwards and forwards in every 
direction, so that the intestine condition of the multitude of these that form the 
medium may be likened to the familiar appearance of a swarm of gnats in a sunbeam. 
The quality of perfect elasticity being common to all the particles, the original 
amount of vis viva, or living, acting force, of the whole multitude must for ever remain 
the same. If undisturbed by external action it cannot, of itself, diminish or increase, 
but must for ever remain as unchanged as the matter that is associated with it and 
that it endows with activity. Such is the case if we view the whole mass of moving 
particles as one object, but each individual of the multitude must at every encounter 
give or receive, according to the ever-changing angle and plane of impact, some portion 
of its force, so that, considered separately, they are for ever continually changing 
the velocity and direction of their individual motions ; striking against and rebounding 
from each other, they run rapidly in their zig-zag conflict through every possible 
mode of concurrence, and at each -point of the medium we may thus conceive that 
particles are moving in every possible direction and encountering each other in every 
possible manner during so small an elapsed interval of time that it may be viewed as 
infinitesimal in respect to any sensible period. The medium must in this way become 
endowed with a permanent state of elastic energy or disposition to expand, uniformly 
sustained in every part and communicating to it the physical character of an elastic 
fluid. 
The simplicity of this hypothesis facilitates the application of mathematics in 
ascertaining the nature and properties of such media, and the study acquires much 
interest from the analogies that it unfolds. For if the reasoning is correct, the 
physical laws common to all gases and vapours—those laws, namely, that concern 
heat and pressure—do actually belong to such media, and may be synthetically 
deduced from the constitution which has now been assigned to them. 
The characteristic which renders a medium susceptible of mathematical treatment is 
