of Magnesium, Copper, and Manganese. 307 



§6. Magnesium. 



The substances examined were metallic magnesium, its 

 chloride, sulphate, and oxide. In the case of the first three 

 of these substances a protecting layer of pure carborundum 

 No. 80 was employed. The oxide, on the other hand, was 

 laid upon the plate as shown in fig. 2, without a covering. 

 In order to prevent the lumps of oxide from moving when 

 in the liquid state three small pieces of graphite were placed 

 round each. The general character of the line spectrum is 

 independent of the nature of the compound used, but on 

 account of the high melting-point of the oxide and the fact 

 that it can be used without a carborundum covering, this 

 substance gives a brighter and better-developed spectrum 

 than the more volatile materials. The relative intensities 

 given in the following table arc founded upon a photo- 

 graphic record obtained in this way with magnesium oxide. 



Single plate furnac 



Air-coal gas flame. 



\. 



3829-35 

 383230 

 3838-29 

 4571-10 

 5167 32 

 5172-68 

 5183-60 



Frin £ 



(thern 



electri 



excitati 



:'e 

 lo- 

 cal 

 on). 



ID 



X 



(") 

 8 



10 



Luminous 

 Vapour 

 (thermo- 

 chemical 

 excitation 

 at 2900° C). 



000 



00 

 



Type. 



tadpole 



long 

 tadpole 





Mantle 



Capacity- 



Cone 



(therm o- 



Spark 



(chemical 



chemical 



(electrical 



excitation). 



excitation 



at 1850°C). 



excitation) 



10 



— 



50 



15 



— . 



50 



20 



— 



100 



5 



5 



3 



7 



— 



12 



8 



— . 



15 



10 



— 



20 



The acting electric field in the vicinity of the plate was 



about 8 — - and the plate temperature, allowing for the 



absence of a carborundum cover, about 3000° C. The 

 relative intensities under the heading- "fringe" refer to that 

 part of the vapour which radiates under the influence of 

 the thermelectronic current, whereas those given under 

 11 luminous vapour " are controlled by the temperature pre- 

 vailing just beneath the fringe, which has been assumed 

 to be about 100° C. less than that of the plate. The scale of 

 intensities is the same as that previously adopted *. Wave- 

 lengths are given in international units. In order to 

 emphasize the significance of the results obtained with the 

 single plate resistance-furnace the relative intensities of the 



* Hemsalech, Phil. Masf. vol. xxxiii. p. 9 (1917). 



