|THE INERT CONSTITUENTS OF THE ATMOSPHERE.|
UNIVERSITY COLLEGE, LONDON.
THE discovery of an element always awakens interest; for the total number of the known elements does not exceed seventy-five, and all the various forms of matter which exist on this globe are necessarily composed of these elements. An element, as is well known, is the ultimate constituent of a compound; and with only a limited number, Nature has provided us with that enormous wealth of minerals, of vegetables and of animals, all of which have as their constituents two or more of these elements.
These elements, however, must not be regarded as isolated entities, each self-dependent, having no relations with its compeers; on the contrary, all the elements exhibit certain connections with their neighbors; and there is to be traced an orderly progression from one class of elements, strongly electro-positive in character, metallic in appearance, very inflammable when heated in the air, and at once attacked by water, to another class, highly electro-negative, transparent, unattackable by oxygen, and without perceptible action on water, through a number of connecting links, each of which serves to soften the transition.
These elements have been arranged in series, and it is by considering the method of arrangement that our interest is awakened. The earliest attempt to make such an arrangement antedates the very idea of the conception of an element. For the division of all matter into metal and non-metal is one which is lost in the mists of antiquity. The word 'metal' is derived from the Greek verb μεταλλάω, I search; and that verb is said to be derived from μέτα and ἅλλα, signifying 'after other things.' As it was recognized that elements are constituents of more complex matter, a conception first emphasized by Boyle, and as the distinction became clear that matter which resists decomposition must be classed as elementary and, after a century and a half, a number of elements were recognized, it was obvious that a number of them might be grouped in classes. Take, for example, the elements chlorine, bromine and iodine, all colored, strongly smelling substances, sparingly soluble in water and forming compounds barely distinguishable from each other in appearance or by a cursory inspection; or take such a group as the metals of the alkalies, lithium.