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POPULAR SCIENCE REVIEW. 
paid to symmetrical formulae than to reactions ; the merit of 
the new consisted in bringing the student once more back to 
nature. 
And the appeal to nature was answered and answered 
abundantly. The new conception of compound radicles was 
rich in results; from it there was developed, — first, the 
theory of types, and subsequently the wider theory of valency, 
which has led to that of atom-linking, and these in their turn 
have reacted on the older and more fundamental notions of 
the science, and have given a new meaning to such terms as 
‘ chemical ’ and ‘ mechanical actions/ ‘ compounds 1 and ‘ mix- 
tures/ &c., while, at the same time, they point the way to the 
chemistry of the future when we shall have gained a definite 
conception of the inner mechanism of the molecule, and of the 
laws which regulate the combinations of molecules in groups, 
and the decompositions of molecules with subsequent forma- 
tions of new atomic systems. 
Let us shortly examine these ideas. If sodium be thrown 
on to water caustic soda is produced, a substance made up of 
hydrogen, oxygen, and the simple radicle sodium ; by another 
reaction a substance can be obtained, consisting of hydrogen, 
oxygen, and the compound radicle nitryl, (H0 2 ). These two 
bodies have analogous formulae, Ha OH and (H0 2 ) OH, they 
may both be regarded as derived from water, H HO, by the 
replacement of one-half of the hydrogen by a radicle ; in one 
case by Ha, in the other by H0 2 . Again the whole of the 
hydrogen in water may be replaced by sodium, with production 
of the compound sodium oxide, Ha 2 0 ; but in many of its 
reactions this compound is the analogue of common ether, 
which is also a compound of oxygen with a (compound) radicle 
Ethyl , and has the formula (C 2 H 5 ) 2 0. How these substances, 
Ha OH, (H0 2 ) OH, Ha 2 0, and (C 2 H 5 ) 2 0, both on account of 
the methods by which they are produced, and because of their 
general reactions, may be classed together as derivatives of 
water, or may be said to belong to the icater-type. Similarly, 
other types have been instituted, and large groups of compounds 
have been brought into the same class as being all referable to 
one parent type. This step in advance is evidently an outcome 
of the theory of compound radicles : without that conception a 
system of classification by types would have been impossible. 
But it was found that while such compound radicles as C 2 H 5 
or H0 2 were capable of replacing but one part by weight of 
hydrogen in water, other compound radicles, such as CO or 
C 2 H 4 , were capable of taking the place of two parts by weight 
of hydrogen. Comparing together these two sets of radicles, it 
might be said that CO —2 H0 2 or C 2 H 4 =2 C 2 H 5 , so far as the 
power of combining with hydrogen was concerned. This 
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