236 



maintaining the change — first from the solid to the fluid state, 

 and secondly from the fluid to the gaseous. The facts are 

 very plain ; a pound of water at the temperature of 0° C, or 

 the freezing point, mixed with a pound of water at 79° yields 

 two pounds at the mean temperature of 39'5°; but a pound of 

 ice or dry snow at the temperature of 0° mixed with a pound of 

 water at 79° yields two pounds of water at 0°, because the 79° 

 of sensible heat in the water are now employed or occupied in 

 maintaining such an amount of vibratory motion in the mole- 

 cules of the icC;, that they are no longer able to obey that poh>r 

 attraction by which they were previously aggregated together 

 in given directions in a crystalline form (for though not so 

 evident in ice, the crystalline character of snow is notorious), 

 and the heat-energy, being thus already occupied in doing 

 work, is incapable of doing any other work, as for example on 

 the organs of sensation, at the same time. The same reasoning 

 applies to the change from the fluid to the gaseous state; but 

 in this case a much larger amount of thermic energy is 

 employed in so far removing the molecules from the sphere of 

 each other's attraction, that the balance of their mutual forces 

 is repulsive, and so long as that repulsion is maintained, the 

 dry steam manifests all the properties common to the fixed 

 gases. " Latent^' heat, then, when properly understood, ceases 

 to be a " stumbling-block to the dynamic theory of heat.'^ 



48. Several quantitative equivalents of energy have been 

 assigned by experiment, but that on which most stress is laid is the 

 equivalence of thermic and kinetic energy. It is a remarkable 

 and unprecedented confirmation of the thermo-dynamic theory, 

 that the numerical results arrived at by three distinct methods 

 of investigation, in the hands of as many independent physicists, 

 should be found to agree within very narrow limits of error. 



49. He must be a bold man who denies that the sun shines at 

 noonday; and scarcely less audacious is the assertion that the 

 experiments of Dr. Joule do not confirm this equivalence. 

 Dr. Joule conducted four distinct series of experiments, three 

 series on the amount of thermic energy produced by molecular 

 friction in stirring respectively water, oil, and mercury; the 

 fourth, on that produced by the friction of two iron surfaces 

 against each other. The four numerical results accorded very 

 nearly, and after assigning to each result its weight, according 

 to its estimated liability to error, he deduced the mean value of 

 772 foot-pounds as the dynamic equivalent of thermic energy."^ 



* For the sake of those who are not already familiar with this subject, it 

 may be stated that a foot-pound is the amount of energy acquired by a 

 weight of one pound in descending through the vertical space of one foot, 



