_High and low temperatures._--The past quarter of a century has witnessed a great development and application of the methods of producing both very low and very high temperatures. Sir James Dewar, by improved apparatus, has produced liquid hydrogen and a fall of temperature probably reaching to the absolute zero. A number of applications of extremely low temperatures to research in various directions has been rendered possible by the facility with which they may now be produced. Similarly high temperatures have been employed in continuation of the earlier work of Deville, and others by Moissan, the distinguished French chemist. _Progress in Chemistry._--In chemistry generally the theoretical tendency guiding a great deal of work has been the completion and verification of the ‘periodic law’ of Mendeléeff; and, on the other hand, the search by physical agents such as light and electricity for evidence as to the arrangement of atoms in the molecules of the most diverse chemical compounds. The study of ‘valency’ and its outcome, stereo-chemistry, have been the special lines in which chemistry has advanced. As a matter of course hundreds, if not thousands, of new chemical bodies have been produced in the laboratory of greater or less theoretical interest. The discovery of the greatest practical and industrial importance in this connection is the production of indigo by synthetical processes, first by laboratory and then by factory methods, so as to compete successfully with the natural product. Von Baeyer and Heumann are the names associated with this remarkable achievement, which has necessarily dislocated a large industry which derived its raw material from British India. [Illustration: FIG. 12. This figure should be examined with a magnifying glass. It is a direct reproduction of a photograph of a detached nebula and surrounding stars in Cygnus by Dr. Max Wolf of Heidelberg (reproduced by permission from the Monthly Notices of the Royal Astronomical Society, vol. lxiv, Plate 18, p. 839, q.v.). The exposure was four hours on July 10th, 1904, with a camera the lenses of which have a diameter of sixteen inches. The picture is enlarged so that the apparent diameter of the Sun or Moon would be about 1⅓ inch on the same scale (one minute, or sixtieth of a degree, equals one millimetre). The “apparent diameter” of the sun or moon is about one in 115: that is to say that a covering disc of any size you like can be made exactly to coincide with and “cover” the disc of the sun or moon provided that you place it at a distance from the eye equal to 115 times its own diameter--thus a disc of an inch in diameter (say a halfpenny) will just “cover” the sun or moon if placed at a distance from the eye of a little less than ten feet, a threepenny piece will cover it at about six feet, and a disc of somewhat less than half that size when held at arm’s length.