466 Scientific Intelligence. 



13. The Radiometer.— Two views have been offered as to the 

 mode of action of the gas in the radiometer. One attributes the 

 motion to _ reaction of gas particles getting heated on the vanes, 



ing off; the other to air currents which arc <Ilr> it. I 

 toward the plate in consequence of heated air rising from it. M. 

 Neesen has endeavored (Pogg. Ann.) to decide between these 

 views. If the second view is correct, he argued, the wall of the 

 vessel, by becoming also heated, must also acquire influence 

 through rise of heated air from it as from the vanes. If the rota- 

 tion be merely a phenomenon of reaction there is no reason to 

 suppose such an influence of the fixed wall. Now by giving the 

 radiometer an eccentric position within the glass vessel such an 

 influence of the walls should be readily recognized. He describes 

 a number of experiments made in this way, and which he regards 

 as supporting the second view. 



In an article contributed to Poggendorff's Annalen, M. Zollner 

 is led to take the following positions in reference to the radiome- 

 ter. The explanation of radiometric motions based on the princi- 

 ples of the mechanical theory of gases, make suppositions about 

 the relation of the mean lengths of path of the gas molecules to 

 the dimensions of the vessel which are not realized in fact. This 

 explanation further leaves out of consideration, without sufficient 

 ground, the simultaneous existence of mercury vapors whose mole- 

 cules have a more then seven times greater mass and a much 

 smaller mean length of path than the molecules of the gases acting 

 35 to the mechanical theory of gases. Hence we are not 

 warranted in regarding the radiometric motion discovered by 

 Crookes as an empirical confirmation of the mechanical theory of 

 gases. — Nature, xv, 521. e. c. p. 



14. Fluorescence. — M. E. Lommel discusses the intensity of the 

 light of fluorescence assuming that tin.- quantity of th< 



an element of volume of the fluorescent substance can emit is pro- 

 portional to the quantity of exciting light which is absorbed. 

 He concludes : 



(1.) That with an increasing concentration, the intensity of the 

 fluorescent light at first increases up to a certain minimum, and 

 then decreases. 



(2.) That the ratio of the light emitted by a superficial layer of 

 kness, to the total fluorescent light, increases with the 

 intensity of the absorption, and is as much more feeble as we 

 observe it at a greater distance. 



(3.) That the mixture of colors which constitutes fluorescent 

 gei with the coefficient of absorption of 

 light, in such a way that the ratio of the intensiti. - 

 more and more that which characterizes the type of the f 

 spectrum, to the extent that the exciting rays are capable of 

 being absorbed. 



(4.) That the fluorescent light, observed at a great distance, is 

 a mixture of colors in which the rays vei 

 absorbed are in less proportion than when the observation is made 

 at a small distance. 



