INDUSTEIAL PROGRESS DURING THE YEAR 1875. \[ x 



eludes with Tyndall that aqueous vapor lias an appreciable ab- 

 sorbing power for heat, though it is much less than Tyndall 

 supposed. The controversy between Tyndall and Magnus 

 was a very natural one, Tyndall, on the one hand, having over- 

 estimated this absorption through neglecting condensation, 

 or vapor-hesion, as Magnus called it ; and Magnus, on the 

 other, having denied its existence because the tube he used 

 was entirely too short to make it apparent with the galva- 

 nometer employed in his experiments. 



OPTICS. 



In Optics, Crookes has published some curious and delicate 

 experiments, in which carefully suspended disks of pith were 

 set in motion in vacuo, apparently by the action of light. 

 In a public lecture, however, given in Edinburgh, Professor 

 Dewar, after explaining the method adopted by Professor 

 Tait and himself for obtaining very perfect vacua by taking 

 advantage of the power that charcoal has of condensing gases, 

 stated that these vacua were so perfect that it was impossible 

 to force through them an electric spark between electrodes 

 one quarter of an inch apart, even when a powerful coil is em- 

 ployed ; and hence that such vacua were, therefore, eminent- 

 ly proper to repeat the investigation recently made by Mr. 

 Crookes upon the action of a beam of light on a disk at the 

 end of a delicately suspended glass fibre. Such an investi- 

 gation has been made by Dewar, and he finds that the move- 

 ments of the disk are due entirely to radiant heat, and not 

 to any mysterious agency, as Mr. Crookes seems to imply. 

 The sensitiveness of the disk increases with the perfection 

 of the vacuum. The sides of the glass receiver must be quite 

 thin. If the disks are covered with lampblack, they are af- 

 fected much sooner than if left white. The conductivity of 

 the suspended body for heat, and the nature of the residuum 

 gas within the vacuum, determine the density of the gas cor- 

 responding to the neutral point observed by Mr. Crookes. The 

 intensity of the movements of the disk increases in proportion 

 to the inverse square of the distance of the source of radiation. 

 If we interpose between the light and the disk a substance 

 opaque to heat rays, although transparent to light, the move- 

 ments of the disk immediately cease. If we interpose a sub- 

 stance transparent to heat, but opaque to light, the deflection 



