390 



Intelligence and Miscellaneous Articles. 



between the limits O'l to 1*5 mm., but it increases a little at higher 

 pressures. At pressures exceeding 1*5 mm. the character of the 

 discharge seems to be modified, after which the radiation increases 

 considerably with the pressure after having passed, at any rate in 

 certain cases, through a minimum. 



3. For the same gas lender the same pressure the composition of the 

 radiation is constant, and does not depend on the intensity of the 

 electrical current. This is subject to the reserve stated in (1). 



4. When the density of the gas changes the composition of the 

 radiation changes also, in so far that the ratio of the intensity of 

 radiation of the shorter ivave-lengths to that of the total radiation 

 decreases with increase of pressure. 



This ratio, indicated in my experiments by the ratio of the 

 radiation transmitted by a plate of alum to that of the total radiation, 

 varies for instance between the pressures 0*1-1'6 mm., for car- 

 bon dioxide from 46 to 15 per cent., and for nitrogen from 94 

 to 60 per cent. 



5. It will be seen from these figures that the ratio between the 

 luminous and the total radiation rises to considerable values for 

 very low pressures, values far greater than those of our ordinary 

 luminous sources. This ratio, however, does not always attain 

 such high values in the electrical discharge in rarefied gases. At 

 pressures of 5-10 mm. it is rather small. 



6. The intensity of the total radiation varies greatly for different 

 gases. It is not in a simple ratio with the molecular weight of 

 the substance, or with the difference of potential in the layer of 

 radiating gas. It does not appear to depend essentially on the 

 absorption of the gas at the ordinary pressures and temperatures, 

 either as regards its intensity or in reference to its composition, 

 as can be seen by the following table, which contains values for the 

 radiation and the difference of potential for a pressure of 0*7 mm. 

 and for the same strength of the electrical current. 





Total. 



Eadiation 



transmitted 



by alum. 



Difference 

 of potential. 



Oxvffen 



1-0 



5-9 



30-0 



13-6 



2-2 



24 



3-8 



17 



19 

 25 



27 



Hvdroeen 



Nitrogen 



Carbon dioxide 





IV. In the case of low pressures it appears to me that we may 

 give the following explanation of these phenomena. 



"When the rarefied gas becomes a conductor of an electrical 

 current, it is the molecules principally which serve to transport 

 : electricity, that is to say the active molecules which radiate. The 



