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stone in a still pond and watching how, as the outward circling 
wavelets spread, they grow weaker and weaker, and if the space 
is large enough, at last seem lost in the calm beyond — this 
affords a good notion to begin with of wave forms and wave 
force ; but suppose, instead of a stone striking the surface of 
water, a sudden explosion took place of a particle of dynamite 
at some depth below. Here we should have waves in all di- 
rections, ascending, descending, and spreading on every side. 
Such waves would bear somewhat the relation to the pond- 
waves that a well-known toy composed of balls within balls 
would do to a mere section of the whole concern. Wave beyond 
wave in consecutive series, spreading in all directions from a 
point, must be conceived as spherical shells one outside the 
other like the coats of an onion, each expanding and contract- 
ing within narrower limits, and sending the wave force and 
the wave form onwards to an indefinite extent. 
The quantities of matter acted upon by these wave forces 
may be very small, and yet the power exerted very great. 
Thus 44 Faraday found the quantity of electricity disengaged by 
the decomposition of a single grain of water in a voltaic cell to 
be equal to that liberated in 800,000 discharges of the great 
Leyden battery of the Royal Institution. This, if concentrated 
in a single discharge, would be equal to a flash of lightning. He 
also estimated the quantity of electricity liberated by the 
chemical action of a single grain of water on four grains of zinc 
to be equal in quantity to that of a powerful thunderstorm.” * 
Tyndall himself also beautifully illustrates this subject in his 
remark : 44 1 have seen snow-flakes descending so softly as not 
to hurt the fragile spangles of which they were composed ; yet 
to produce from aqueous vapour a quantity which a child 
could carry of that tender material, demands an exertion of 
energy competent to gather up the shattered blocks of the 
largest stone avalanche I have ever seen, and pitch them twice 
the height from which they fell.”f 
When galvanic electricity is employed to decompose water, 
the constituent elements, oxygen and hydrogen, are not merely 
allowed to separate, but they are pulled apart with great 
force, and Grassiot showed that if the water were confined in iron 
bottles an inch thick, a small battery gave force enough to 
split them asunder. The directive force which the particles of 
water obey in the act of freezing, and which leads to its ex- 
pansion, has similar power ; and, as is well known, a very small 
quantity of water will burst a strong shell or split a great rock. 
Although heat sets the molecules of all sorts of matter in 
* Tyndall, u Notes on Electricity,” p. 15. 
t “Heat as a Mode of Motion,” 4th. edit. p. 147. 
