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 Rh thinking and to the greatest accuracy in the observation of minute phenomena. He appeared to be destined to disclose to mankind views into many hitherto hidden depths of his nature. I have keenly felt the disappointment of hope caused by his death, for Hertz was the one of my students who entered most fully into my own circle of scientific thoughts and on whom I most confidently relied for their future development. He has by his discoveries secured a permanent fame in science. Not only will his name live through his labors, but his lovely, noble traits of character, his uniform modesty, his glad recognition of the merit of others, the gratitude he felt toward his teacher, will never be forgotten by any who knew him. His only thought was for the truth, which he sought with extreme earnestness and all his might. He was never moved by ambition or self-interest. Even when he had a right to claim discoveries for himself, he was rather inclined to refrain. While usually quiet and taciturn, he could take an animated part in the social circle of his friends, and enliven the conversation by many a pertinent word. He never had a personal adversary, but he could on occasion utter a sharp judgment upon slovenly work or notoriety-seeking efforts that gave themselves out for science. How fully his thoughts embraced the widest views of science is illustrated in this book, the last monument of his earthly effort, in which he has sought to give a consecutive presentation of a system of mechanics consistent in itself, and to deduce all the special laws of that science from a single principle. Great difficulties are indeed still to be overcome in the effort to explain single sections of physics from the principles developed by Hertz. But as a whole his treatise must interest in the highest degree every reader who can enjoy a consistent system of dynamics presented in the most complete and comprehensive mathematical setting.



N the 16th of February, 1894, Prof. Ernst Haeckel, the most eminent representative of natural science and the most ardent advocate of the doctrine of evolution in Germany, celebrated his sixtieth birthday and received the congratulations of numerous friends and pupils from far and near, who in many cases emphasized the expression of their good wishes by the presentation of appropriate gifts. Of these tokens of friendship and esteem perhaps the most suitable, as well as the most striking,