lx GENERAL SUMMARY OF SCIENTIFIC AND 



of the disk is larjxc. If two disks are taken, one of rock-salt 

 and the other of glass, it is found that the rock-salt is inactive 

 when a beam of light is thrown on it, but the glass disk is 

 active, the reason being that the rock-salt is not heated, 

 whereas the s;lass is heated. To show the sensitiveness of 

 the apparatus, it may be stated that an ordinary lucifer- 

 mateh will, at a distance of four feet, produce instant action. 

 Professor Dewar has not accepted the suggestion of Reyn- 

 olds that the action is due to the evaporation of some of the 

 fluid on the surface of the disk. These phenomena allow of 

 a very perfect explanation, according to the principles of the 

 kinetic theory of gases and the mechanical theory of heat, 

 according to which the particles of gas are flying about in 

 all directions with a velocity which depends upon their tem- 

 perature ; the length of the path of each particle is depend- 

 ent principally upon the barometric pressure. Under ordi- 

 nary barometric pressure of thirty inches, the length of the 

 average path is about one ten-thousandth of a millimeter, 

 but when the barometric pressure is reduced to the one- 

 millionth part of an inch, the average length of the path be- 

 tween two collisions is about eighteen inches. If, therefore, 

 Mr. Crookes's disks are in such a vacuum, and heated by ra- 

 diation on one side warmer than on the other, the particles 

 of gas that impinge on that side of the disk leave it at a 

 higher temperature, and therefore with a greater velocity, 

 than those striking the opposite side. Hence there is a re- 

 coil of the disk as observed by him. 



AVibel has made additional experiments upon the cause of 

 the luminosity of flames. He finds the results of Knapp con- 

 firmed, that nitrogen, hydrogen chloride, carbon dioxide, and 

 other indifferent gases, act like air to destroy the luminosity 

 of gas used in a Bunsen burner; but he also finds, curiously 

 enough, that this luminosity may be wholly or partially re- 

 stored by heating the tube to redness through which the 

 mixture passes. Hence he concludes, 1st, that the absence 

 of luminosity in a Bunsen flame is not due to dilution of the 

 gas; 2d, that it is due to the cooling effect of the inert gas, 

 since, if this be heated, the luminosity returns; 3d, that the 

 luminosity of aflame depends upon the temperature existing 

 in its interior; and, 4th, that ordinary illuminating materials 

 are such because the rising gases and vapors are sufficiently 



