Chemiluminescence 439 



1st, It becomes luminous in oxygen gas only at a temperature of about 

 16° to 22° of Reaumur. 



2dly, Of all the non-respirable gases as pure as possible, it is luminous 

 only in azotic gas, oxidated azotic gas, and muriatic acid gas. 



3dly, In muriatic acid gas it inflames immediately of itself, and burns 

 with great brightness. 



4thly, The light of phosphorus is stronger in rarefied air. 



5thly, It emits no light in vacuo. 



6thly, On being subjected to heat, it inflames and burns with rapidity 

 in oxygen gas; and in the non-respirable gases, not pure, its light is 

 stronger. 



Vthly, By its phosphorescence in oxygen gas no carbonic acid is formed. 



8thly, When artificial phosphorus has emitted light in the nonrespir- 

 able gases not perfectly freed from oxygen gas, a fresh piece of phos- 

 phorus does not become luminous in them: azotic gas, however, is an 

 exception, in which, after being purified, phosphorus becomes luminous 

 for some time. 



9thly, Moisture and wet are impediments to its being luminous. 



lOthly, Fluids are altogether contrary to the luminous property of 

 artificial phosphorus. 



In his comparison with shining wood (see Chapter XIV) , Broeck- 

 man found some differences, especially in the conditions necessary 

 for phosphorus to become luminous, and therefore held that the 

 assertion of Spallanzani, that the greatest analogy exists between the 

 luminous phenomena of wood and of phosphorus, must lose some 

 of its weight. 



Some of Boeckmann's statements are correct and some are wrong, 

 owing to the use of impure gases. As indicated in the first conclu- 

 sion, temperature, noticed by Lemery many years ago, is most im- 

 portant, and other conditions (cf. conclusion 9) greatly affect the 

 luminescence. Only through a very large number of subsequent 

 workers was the fact established that a certain minimum pressure 

 of oxygen is necessary for luminescence and in addition there is a 

 " maximum luminescence pressure " of oxygen above which no lumi- 

 nescence occurs. The exact values for these critical pressures depend 

 in a complicated manner on temperature, water vapor, and presence 

 of quenching impurities. Such relations are responsible for the para- 

 doxical fact that sometimes phosphorus appears to luminesce with 

 greater intensity as the oxygen pressure is lowered, mentioned by 

 Boeckmann in conclusion 4. 



Perhaps the most extraordinary finding of the eighteenth century 

 is that phosphorus does not luminesce in 100 per cent oxygen at 

 ordinary temperatures. This observation appears to have been made 

 as early as 1788 by A. F. de Fourcroy, and has been confirmed by 



