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POPULAR SCIENCE REVIEW. 
there exists in nature two agencies, matter which is pon- 
derable, visible, and tangible, and a something which is 
imponderable, invisible, and appreciable only by its influence 
on matter, Metcalfe maintains that the imponderable and 
active agency, which he calls “ caloric,” is not a mere form 
of motion, not a vibration amongst the particles of ponder- 
able matter, but itself a material substance flowing from the 
sun through space, filling the voids between the particles of 
solid bodies and conveying by sensation the property called 
heat. The material nature of caloric, or sun force, is con- 
tended for by him on the following grounds : — 
1. That it may be added to, and abstracted from, other 
bodies, and measured with mathematical precision. 
2. That it augments the volume of bodies which are again 
reduced in size by its abstraction. 
3. That it modifies the forms, properties, and conditions of 
all other bodies. 
4. That it passes by radiation through the most perfect 
vacuum that can. be formed, in which it produces the same 
effects on the thermometer as in the atmosphere. 
5. That it exerts mechanical and chemical forces which 
nothing can restrain, as in volcanos, the explosion of gun- 
powder and other fulminating compounds. 
6. That it operates in a sensible manner on the nervous 
system, producing intense pain; and, when in excess, dis- 
organization of the tissues. 
As against the vibratory theory, Metcalfe further argues 
that, if caloric were a mere property or quality, it could not 
augment the volume of other bodies ; for this purpose it must 
itself have volume, it must occupy space ; and it must, there- 
fore, be a material agent. If caloric were only the effect of 
vibratory motion amongst the particles of ponderable matter, 
it could hot radiate from hot bodies without the simultaneous 
transition of the vibrating particles ; but the fact stands out 
that heat can radiate from material ponderable substance 
without loss of weight of such substance. 
“Ho metaphysical subtlety,” he adds, “can refute the 
belief of mankind that whatever operates in a sensible manner 
upon material organs must be a material substance, for the 
obvious reason that there can be no virtue without substance, 
as maintained by Hewton.” 
With this view as to the material nature of caloric, or sun 
force; with the impression firmly fixed on his mind that 
“everything in nature is composed of two descriptions of 
matter, the one essentially active and ethereal, the other 
passive and motionless,” Metcalfe based the hypothesis that 
the sun force, or caloric, is a self-active principle. For its own 
