According to the lemniscate arrangement, we should commence with hydrogen
as the head of the first negative group, but as it differs wholly from
those placed with it, it is better to take it by itself. Hydrogen is the
lightest of the known elements, and is therefore taken as 1 in ordinary
chemistry, and all atomic weights are multiples of this. We take it as 18,
because it contains eighteen ultimate atoms, the smallest number we have
found in a chemical element. So our "number-weights" are obtained by
dividing the total number of atoms in an element by 18 (see p. 349,
January).
[Illustration: PLATE V.]
HYDROGEN (Plate V, 1).--Hydrogen not only stands apart from its reputed
group by not having the characteristic dumb-bell shape, well shown in
sodium (Plate I, opposite p. 349, January), but it also stands apart in
being positive, serving as a base, not as a chlorous, or acid, radical,
thus "playing the part of a metal," as in hydrogen chloride (hydrochloric
acid), hydrogen sulphate (sulphuric acid), etc.
It is most curious that hydrogen, oxygen and nitrogen, the most widely
spread gases, all differ fundamentally in form from the groups they
reputedly head.
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