G8 
ALEX. W. STEIN. 
destructibility of force is well illustrated iu the example of latent 
heat. If a pound of water at 32° be mixed with a pound at 
174°, the mixture will have the mean temperature—that is, 103°; 
but if a pound of ice at 32° be mixed with a pound of water at 
174°, the mixture still remains at 32°, and the reason for this is 
that the solid (ice) in passing into a liquid (water) requires 142° 
of heat. This heat is inappreciable by the thermometer. The 
heat energy is spent in overcoming the force of cohesion. But it 
is not lost, for when the liquid (water) again assumes the solid 
(ice) state, it is again evolved as appreciable heat. A wise pro¬ 
vision that time is necessary for the melting of snow and ice, on 
the one hand, and the freezing of water on the other. Coal and 
wood represent the amount of force in the shape of the light and 
heat of the sun which was expended in their production, and when 
they are burnt give out again just that amount—a conversion of 
first visible energy into potential energy, and potential into vis¬ 
ible energy,—a resurrection, as it were, of force buried for cen¬ 
turies beneath the ground. 
So I might show the reciprocal relations of the physical forces 
by chemical action producing electricity, electricity producing 
heat, light and motion. Thus we know the physical forces by 
their effects; we know that all bodies, with a few exceptions, 
expand when the temperature is increased, and contrary when it 
is reduced. We know that there is an affinity exerted between 
minute particles of different kinds of matter, causing them to 
combine so as to form new bodies endowed with new properties. 
This we term electrical affinity, but there our knowledge ends. 
But you may say, the trouble is that the forces of cohesion and chem¬ 
ical affinity act through such inconceivably small intervals ; but 
you are no better off if you take the force that acts at a great dis¬ 
tance, viz., gravitation. You say the sun pulls the planets be¬ 
cause it has 700 times more molecules than all the planets put to¬ 
gether, and so it keeps them moving around it. Our earth has 
eighty times more molecules than our moou, and so it keeps the 
moon moving around it. But you cannot tell why an apple falls 
to the ground, except on the same unsatisfactory hypothesis. 
I cite these more than familiar examples, and I am afraid you 
