6 
MR, J. J. WATERSTON ON THE PHYSICS OF MEDIA COMPOSED OF 
portion of its weight a like theory of the phenomena of heat ; and we are, perhaps, 
justified in expecting that the complete development of this theory will have a much 
more important influence on the progress of science, because of its more obvious 
connection and intimate blending with almost every appearance of Nature. Heat is not 
only the subject of direct sensation and the vivifier of organic life, but it is manifested 
as the accompaniment of mechanical force. It is related to it both as cause and effect, 
and submits itself readily to measurement by means of the mechanical changes that 
are among the most prominent indications of its change of intensity. The undulatory 
theory at once leads us to the conclusion that, inasmuch as the temperature of a body 
is a persistent quality due to the motion of its molecules, its internal constitution 
must admit of it retaining 1 a vast amount of living force. Indeed, it seems to be 
almost impossible now to escape from the inference that heat is essentially molecular 
vis viva. In solids, the molecular oscillations may be viewed as being restrained by 
the intense forces of aggregation. In vapours and gases these seem to be overcome ; 
vibrations can no longer be produced by the inherent vis insita of the molecules 
struggling with attractive and repellant forces ; the struggle is over and the molecules 
are free; but they, nevertheless, continue to maintain a certain temperature; they 
are capable of heating and being heated; they are endowed with the quality heat, 
which, being of itself motion, compels us to infer that a molecule in motion without 
any force to restrain or qualify it, is in every respect to be considered as a free pro¬ 
jectile. Allow such free projectiles to be endowed with perfect elasticity, and 
likewise extend the same property to the elementary parts of all bodies that they 
strike against, and we immediately introduce the principle of the conservation of 
vis viva to regulate the general effects of their fortuitous encounters. Whether gases 
do consist of such minute elastic projectiles or not, it seems worth while to enquire 
into the physical attributes of media so constituted, and to see what analogy they 
bear to the elegant and symmetrical laws of aeriform bodies. 
Some years ago I made an attempt to do so, proceeding synthetically from this 
fundamental hypothesis, and have lately obtained demonstration of one or two points 
where the proof was then deficient. The results have appeared so encouraging, 
although derived from very humble applications of mathematics, that I have been 
led to hope a popular account of the train of reasoning may not prove unacceptable to 
the Royal Society.—Sept. 1, 1845. 
Section I. —Of a Homogeneous Medium and the Laws of its Elasticity. 
§ 1. The term medium is, perhaps, not quite appropriate to what is here intended 
to be signified. We speak of a resisting medium, of the medium of light, and in each 
expression something is referred to as intervening between bodies, and it is the 
quality of interposition that entitles it to the name. Here, for want of better, it is 
employed to denote a certain hypothetical condition of matter which it is the object 
of this Paper to show has physical properties that resemble those that have been 
