Such speculations as these lead us far afield, into the realm of mathematics, metaphysics and ultimate realities. Even the most material of all the sciences--chemistry--leads thither when pushed to its final analysis. The visible, sensible universe vanishes, and is replaced by the invisible, the super-sensible. Yet science has been our guide throughout. William James once remarked that metaphysics is merely “persistently clear thinking.” It endeavors to find the ultimate causes of things, the _noumena_ behind phenomena, the reality behind appearances. The physical world in which we live is a world of phenomena only; real in a sense, and for all practical purposes, and yet the greatest of all unrealities in another sense. It is a mere world of appearances; a phantasmagoria of fleeting shapes and shadows. We feel that reality must exist somehow, somewhere; yet we can never find it. We can no more find it by chemical analysis than we can discover the mind and soul of man by dissecting the brains of corpses,--or even by vivisection! Something always escapes us--the Soul of Things, the Ultimate Reality, the Great Unknown. Such thoughts and speculations as these, however, need not occupy the mind of the practical chemist. For him, atoms exist, so do elements, so does “matter.” For practical, daily life, we certainly have to live _as if_ matter existed, and the chemist has to proceed with his work upon the assumption that matter actually does exist--it is “real.” Certain it is that the practical furtherance and application of this science can come about in no other way. Chemistry has revolutionized our lives; it has penetrated all fields of commerce and industry, and its practical application has rendered possible and pleasant the lives of countless thousands of persons now living upon our planet. We owe more to chemistry than we can ever repay--or rather to those brilliant and unselfish men who have built up the modern science of chemistry. It is my hope that the present little book may in some degree have helped to emphasize this fact. Transcriber’s Notes Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in the original book; otherwise they were not changed. Simple typographical errors were corrected; unbalanced quotation marks were remedied when the change was obvious, and otherwise left unbalanced. This book uses both “reactions” and “reäctions”. Page 17: “Cobalt” was printed as “Cobolt”; changed here. Page 23: The “constitutional” formula in the original book was printed with the two “B”s stacked, one above the other, and in a smaller font that could fit both of them on one line of normal-sized text. Page 49: “allotrophic” was printed that way; probably should be “allotropic”.