is our sole test of the very existence of matter; it is the balance which has proved that nothing ever disappears.  Imponderable matter is no more possible than a triangular ellipse.  Away, then, with such a mischief-breeding conception!  Let this last-surviving fetich be ousted from the fair temple of inorganic science.  Undulations have been measured and counted; quantitative relations, like those expressed in Joule’s law, have been established between them; but an “ether” has never yet been the object of human ken.

We have expressed ourselves thus emphatically upon this all-important point, in order to warn the reader of Dr. Youmans’s book against drawing conclusions which the author himself evidently does not mean to convey.  No clear ideas can ever be entertained in physics until this anomalous “ether” is excommunicated; and therefore we wish it had been banished from this excellent treatise.  We differ also very widely from the author’s views of animal heat, but have not space to enter upon the discussion.  With these exceptions we know of nothing in the work that could be improved.  It is an honor to American science, and fully merits a more exhaustive examination than we have here been enabled to bestow upon it.

Strategy and Tactics.  By General G.H.  DUFOUR, lately an Officer of the French Engineer Corps, Graduate of the Polytechnic School, and Commander of the Legion of Honor; Chief of Staff of the Swiss Army.  Translated from the latest French Edition, by WILLIAM R. CRAIGHILL, Captain U.S.  Engineers, lately Assistant Professor of Civil and Military Engineering and Science of War at the U.S.  Military Academy.  New York:  D. Van Nostrand.

The author of this work is a distinguished civil and military engineer and practical soldier, who, in all military matters, is recognized as one of the first authorities in Europe.  His history is especially interesting to Americans, since not many years ago he played a prominent part in the suppression of a rebellion which, in many features, exhibited a remarkable similarity to the one with which our own Government is contending.  We refer to the secession of the seven Swiss cantons forming the Sonderbund, which, like the insurrection of the Southern States, was a revolt of reactionary against liberal principles of government; it was likewise the fruit of a well-organized and long-matured conspiracy, which only delayed an open outbreak until all its preparations were adequately perfected for a formidable resistance.  The issue of the contest was what we may hope will be that of our own,—­the triumph of free principles, and the complete reestablishment of the authority of the legitimate Government on a firmer basis than it had before occupied.