216 FRAGMENTS OF SCIENCE. 
while the latter is Physics applied to atoms and molecules. 
The subjects of Physics proper are therefore those which 
lie nearest to human perception: light and heat, color, 
sound, motion, the loadstone, electrical attractions and 
repulsions, thunder and lightning, rain, snow, dew, and so 
forth. Our senses stand between these phenomena and the 
reasoning mind. We observe the fact, but are not satisfied 
with the mere act of observation: the fact must be 
accounted for fitted into its position in the line of cause 
and effect. Taking our facts from Nature we transfer 
them to the domain of thought: look at them, compare 
them, observe their mutual relations and connections, and 
bringing them ever clearer before the mental eye, finally 
alight upon the cause which unites them. This is the last 
act of the mind, in this centripetal direction in its prog- 
ress from the multiplicity of facts to the central cause on 
which they depend. But, having guessed the cause, we 
are not yet contented. We set out from the center and 
travel in the other direction. If the guess be true, certain 
consequences must follow from it, and we appeal to the 
law and testimony of experiment whether the tiling is so. 
Thus is the circuit of thought completed from without 
inward, from multiplicity to unity, and from within 
outward, from unity to multiplicity. In thus traversing 
both ways the line between cause and effect, all our reason- 
ing powers are called into play. The mental effort involved 
in these processes may be compared to those exercises 
of the body which invoke the co-operation of every muscle, 
and thus confer upon the whole frame the benefits of 
healthy action. 
The first experiment a child makes is a physical experi- 
ment: the suction-pump is but an imitation of the first 
act of every new-born infant. Nor do I think it calcu- 
lated to lessen that infant's reverence, or to make him a 
worse citizen, when his riper experience shows him that 
the atmosphere was his helper in extracting the first 
draught from his mother's breast. The child grows, but 
is still an experimenter: he grasps at the moon, and his 
failure teaches him to respect distance. At length his 
little fingers acquire sufficient mechanical tact to lay hold 
of a spoon. He thrusts the instrument into his mouth, 
hurts his gums, and thus learns the impenetrability of 
matter. He lets the spoon fall, and jumps with delight to 
