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this, he coolly overthrows the law of gravitation — supposing it 
to be intermittent and uncertain in its operation ! 
Gravitation was nothing to Lucretius, when once mounted on 
his waxen wings, although like Icarus, — 
“ ceratis ope Daedalea 
Nititur pennis, vitreo daturas 
Nomina ponto.”* 
In like manner Professor Tyndall passes from the regions of 
the chemical to those of the structural forces, taking leave of 
all caution when once he has abandoned the reins to his “ scien- 
tific imagination.” 
“ It is instructive to note from this point of view the successive introduc- 
tion of new conceptions. The idea of the attraction of gravitation was 
preceded by the observation of the attraction of iron by a magnet, and of 
light bodies by rubbed amber. The polarity of magnetism and electricity 
appealed to the senses, and thus became the substratum of the conception 
that atoms and molecules are endowed with definite attraction and repellent 
poles, by the play of which definite forms of crystalline architecture are 
produced. Thus, molecular force becomes structural t 
Does the Professor mean to say that “ molecular force ” is 
the same with chemical affinity, and that chemical affinity is the 
same with electricity and magnetism, and also with gravitation ? 
— that we have thus safely reached the brink of an abyss over 
which we take a fortunate leap in the next sentence, and solve the 
great problem, landing safely in the hitherto unknown region of 
the forces which govern organization? The pace takes away 
the breath ; but let us at all events look before we leap. 
“ It requires no great boldness of thought to extend its play into organic 
nature, and to recognize in molecular force the agency by which both plants 
and animals are built up ! In this way out of experience (?) arise concep- 
tions which are wholly ultra-experiential.” f 
For this last admission I am thankful, and for the elegant 
words in which it is clothed. 
We can understand, in the first place, that “an atom is the 
smallest quantity of an element indivisible by chemical means, 
which can exist in a simple body ; and, in the second place, 
that a molecule is a group of atoms forming the smallest 
quantity of a simple or compound body which can exist in a 
free state, or is able to take part in, or result from a reaction.” J 
But no boldness of thought can extend the play either of 
atoms or groups of atoms, that is, molecules, into the pro- 
duction of organic structure. This conception is unthinkable , 
* Horace, Book iv. Ode 2. t Address, p. 52. 
J An Introduction, &c. By Dr. A. 0. Wurtz, F.R.S., pp. 33, 34. 
