It is a fact not without significance, that the proposal of this enterprise was received with the most cordial favour by the eminent scientific men who were solicited to aid in carrying it forward. Most of them consented at once; but, while some were so heavily burdened with work that they could enter into no immediate engagements, not one of them declined to coöperate, and all promised to do so at the earliest practicable opportunity. The feeling of the desirableness of such an undertaking was strong and unanimous. The old dislike of the cultivators of science to participate in the work of popular teaching, seems very much to have passed away; and in England, France, and Germany, alike it was freely acknowledged that _savants_ have an imperative duty to discharge in relation to the work of general scientific education. As remarked by Prof. Virchow, of Berlin, "the destiny of science is the service of humanity." It was stipulated by the authors that they should have ample time for the preparation of their books, and, as the arrangements were recently made, only a few of the works are yet ready. Several, however, are now in press, and will shortly appear. Those interested in the series are under many obligations to Prof. Tyndall for his kindness in consenting to furnish its commencing volume. Being prepared in a short time, amid great pressure both of laboratory and literary work, it contains somewhat less matter than may be expected in the ensuing volumes. It treats of subjects upon which he is perhaps the highest living authority; and it is an admirable example of that vivid, stirring, impressive style for which its author is so distinguished. Prof. Tyndall is not only a master in the "scientific use of the imagination," but in kindling the action of that faculty in his readers. He writes in pictures, so as to make them see what he sees. In this volume he addresses himself directly to his juvenile friends, groups them around him, takes them with him to his favourite mountains, and thus adds a dramatic element and the effect of personal sympathy to familiar colloquial exposition. The "International Scientific Series" will form an elegant and valuable library of popular science, fresh in treatment, attractive in form, strong in character, moderate in price, and indispensable to all who care for the acquisition of solid and serviceable knowledge; and it is commended to American readers as a help in the important work of sound public education. E. L. Y. New York, _September, 1872_. AUTHOR'S PREFACE.