117_a_. HAMMER AND FLEMING’S MOLECULAR SCIAGRAPH, WITHIN A VACUUM TUBE. (_Citations below._)—In view of the overwhelming evidence concerning the generation of X-rays by the impact of cathode rays, within a high vacuum upon the glass or material which preferably forms the anode, it becomes appropriate, it is thought, to review the state of this department of science, in order to arrive a little more closely at the relations which exist between phenomena of low and high vacua. With the former, in that condition in which striae are formed, permanent black bands or deposits are produced upon the surface of the glass; the motion of the particles, therefore, appearing to be in planes at right angles to the line joining the anode and cathode. § 40. That the striae should touch the walls of the tube seems to be necessary for the production of the deposit. § 44. With a high vacuum, the direction of the cathode rays may be any that one desires, it being only necessary to shape the cathode properly, on the principle that the rays radiate normally from the surface. It is known that the radiation is normal as much from the position of the deposit as from that of the phosphorescent spot. It is certain that they are rectilinear. § § 57 and 58. The phosphorescent spot becomes always, sooner or later, when occurring upon the same part of the glass, the location of a deposit from the cathode (§ 123), even when the cathode is aluminum. § 123. The deposit is not the cause of the fatigue of the glass. § 58. Puluj verified this. A wheel was made to rotate by the radiations from the cathode, and therefore it is highly probable that the motion of the molecules, which caused the deposit, is the force that made the wheel rotate. § 58_a_. Why does it not follow that with increase of E. M. F. the particles are thrown with such rapidity that upon striking the proper surface (§ 80), X-rays are generated, but that they are not generated when the velocity of the molecules is insufficient. § 61_b_, p. 46. Attention is now invited to a phenomenon which illustrates that a permanent sciagraph of objects may be impressed upon the inner surface of a vacuum tube, by the deposit of molecules of one of the electrodes. Refer, therefore, to the figure on page 30, “Hammer and Fleming’s Molecular Sciagraph.” As will be seen from further explanation and from the picture itself, the sciagraph _a b_ is made because of the projection, in rectilinear lines, of molecules of carbon or metal, from one of the electrodes, or at least from one more than the other. One leg of the carbon, being in the way of the other, causes a less deposit to be produced upon the glass at the intersection of the plane of the horse-shoe filament and the wall of the vacuum tube. Electrodes exist because the filament is of such a high resistance as to produce a difference of potential between the two straight lower portions of the filament. Mr. William J. Hammer possesses a remarkable