We also notice that it is very difficult, if not impossible, for the inquisitive scientist to limit himself strictly to the object-matter of his own chosen field, and not to enquire more or less earnestly—not infrequently to dogmatize more or less positively—respecting the results of other sciences and even respecting the foundations and processes of scientific inquiry itself. Thus Mr. Huxley in the first Essay of this Series on _The Physical Basis of Life_, leaves the discussion of his appropriate theme in order to deliver sundry very positive and pronounced assertions respecting the “limits of philosophical inquiry,” and quotes with manifest satisfaction a dictum of David Hume that is sufficiently dogmatic and positive, as to what these limits are. In more than one of his Lay sermons, he rushes headlong into the most pronounced assertions in respect to the nature of matter and of spirit. The eloquent Tyndall, in No. 5, expounds at length _The Methods and Tendencies of Physical Investigation_ and discourses eloquently, if occasionally somewhat poetically, of _The Scientific use of the Imagination_. But Messrs. Huxley and Tyndall are eminent examples of scientists who are severely and successfully devoted respectively to physiology and the higher physics. No one will contend that they have not faithfully cultivated their appropriate fields of inquiry. The fact that neither can be content to confine himself within his special field, forcibly illustrates the tendency of every modern science to concern itself with its relations to its neighbors, and the unresistible necessity which forces the most rigid physicist to become a metaphysician in spite of himself. So much for the appellation “_Scientists_.” “_Half Hours_” suggests the very natural inquiry—What can a scientist communicate in half an hour, especially to a reader who may be ignorant of the elements of the science which he would expound? Does not the phrase _Half Hours with Modern Scientists_ stultify itself and suggest the folly of any attempt to treat of science with effect in a series of essays? In reply we would ask the attention of the reader to the following considerations. The tendency is universal among the scientific men of all nations, to present the principles of science in such brief summaries or statements as may bring them within the reach of common readers. The tendency indicates that there is a large body of readers who are so far instructed in the elements of science as to be able to understand these summaries. In England, Germany, France and this country such brief essays are abundant, either in the form of contributions to popular and scientific journals, or in that of popular lectures, or in that of brief manuals, or of monographs on separate topics; especially such topics as are novel, or are interesting to the public for their theoretic brilliancy, or their applications to industry and art.