512 HISTORY OF SCIENCE. 



The apparatus used by Dr. Tyndall in his researches on the dia- 

 thermancy of gases consisted of a metallic tube 3 inches in diameter 

 and 4 feet long. The ends of the tube were closed with parallel plates 

 of rock-salt (see page 509). A Leslie's cube (page 507), or other source 

 of heat, was placed at one end of the tube, the thermo-pile, with its 

 cone, at the other. The air could be exhausted from the interior tube 

 to any required degree, and various gases and vapours introduced as 

 required. The radiation through the tube was usually balanced by a 

 constant source of heat, so adjusted on the other side of the thermo- 

 pile that the needle of the galvanometer was brought to zero. The 

 effect of any change in the radiation by the introduction of substances 

 into the tube was to destroy this balance and produce a deflection. 

 Some of the results obtained by Dr. Tyndall are very interesting and 

 instructive. The space at our command will not admit of their enu- 

 meration. The following, however, may be specified, as they are 

 important in the economy of nature. Dry air permits radiant heat to 

 pass as freely as a vacuous space ; but aqueous vapour, if present even 

 in very small proportion, powerfully arrests the passage of radiant heat. 



When Melloni found that the obscure rays were in a great measure 

 intercepted by glass, while rock-salt permitted all rays to pass without 

 impediment, he caused to be constructed a prism and a lens of rock-salt, 

 with which he repeated Sir W. Herschel's experiments on the heating 

 rays of the solar spectrum. He found the heating effect feeble in the 

 violet, and gradually increasing to the red end of the spectrum. Be- 

 yond the red it continues to increase, the maximum is at a point out 

 of the visible spectrum, and the intensity then diminishes. The in- 

 visible part of the calorific spectrum is but little inferior in extent to 

 the visible spectrum. 



But if there are invisible rays beyond the red end of the spectrum, 

 there are also invisible rays beyond the violet end. Scheele early in 

 1781 had noticed that chloride of silver is more blackened by the 

 violet than by the red end of the spectrum, and Wollaston (page 480) 

 discovered that this action extended beyond the visible violet. There 

 are therefore in the solar spectrum rays more refrangible than the last 

 visible gleam of violet, and it is precisely these ultra-violet rays which 

 exert the most powerful chemical actions. These are the rays which 

 are most active in producing photographic impressions, and it is indeed 

 only by this chemical activity that we are acquainted with their exis- 

 tence. The ultra-violet part of the solar spectrum has, like the visible 

 part, numerous gaps, for it is crossed by an immense number of lines. 

 Many hundreds of these are visible in a good heliograph of the ultra- 

 violet spectrum. And as they are to the chemical spectrum what 

 Fraunhofer's lines (page 480) are to the visible spectrum, E. Becquerel 

 has proposed to extend Fraunhofer's nomenclature to the most marked 

 ultra-violet lines by assigning to them the letters L, M, N, o, P, Q, R, s, T. 

 Diagrams have been constructed like that in Fig. 233 to show the 



