Elastic Force, Density, and Temperature in Gases* 53 



nexion of the ponderable and imponderable elements of the uni- 

 verse. We are indeed striving to carry the domain of the induc- 

 tive experimental philosophy into the properties of the subtile 

 matters called, often, the imponderables, and searching for the 

 causes of the development of mechanical force in the actions, 

 reactions, affinities, incompatibilities, &c. of these subtile agents, 

 which are struggling to pass from the state of instability which 

 has been impressed upon them by an omnipotent power to a state 

 of stable equilibrium, through the properties which they possess. 

 The phenomena of the actions of these properties in their inde- 

 finitely varying states constitute the phenomena of nature ; and 

 in investigating them we are striving to attain the highest degree 

 of knowledge within the reasoning powers which have been con- 

 ferred upon us. We must therefore be content with the progress 

 of knowledge which the inductive philosophy affords to us, and 

 leave wild dreams and speculations to the day-dreamers and spe- 

 culators, and receive as probable only such theories and hypo- 

 theses as have an experimental basis, and for the probability of 

 which good reasons can be advanced. 



From the time of the discoveries of Dr. Black in heat, about a 

 century ago, the advance of our knowledge has been great, 

 although apparently slow, and often deviating from a straight 

 course. The properties now proposed to be discussed are amongst 

 those which require peculiar watchfulness of our instruments, as 

 changing properties are to be learnt, and evanescent results are 

 to be noted. 



It has long been known that, in popular language, high-pres- 

 svre steam blows cold. Now this means that a jet of steam from 

 a boiler in which it exists of high temperature and elastic force, 

 on being allowed to escape through an aperture into the air, soon 

 loses its high temperature and gives a feeling of cold to the hand 

 on which it strikes. The sensible heat of the steam has dimi- 

 nished, because its capacity for caloric has increased by its ex- 

 pansion in volume and diminution of elastic force. It is now 

 the question for consideration whether the change of capacity for 

 caloric is instantaneous on the sudden escape of the steam from 

 a high to a low pressure. We see that the change is not instan- 

 taneous, but is developed in time; for on bringing the hand or 

 a thermometer nearer to the aperture from which the steam 

 issues, the sensible heat increases, and the capacity in the jet of 

 course diminishes, so that we must conclude that the change of 

 the capacity for caloric is developed only in some time after the 

 steam is relieved from the pressure balancing its elastic force in 

 the boiler. To determine this time is a very important point in 

 the theory of the mechanical force of steam, and its applications 

 in the steam-engine. The corresponding properties of air are 



