12 Dr. Karl Heumann's Contributions to 



greater brilliancy than when the combustion proceeds in 

 ordinary air. The product of combustion is in this case also 

 a solid, viz. stannic oxide. 



In order to prove that a similar appearance is noticeable in 

 the case of luminous vapours, in so far as these are not oxi- 

 dized to non-luminous gases by excess of oxygen, hydrogen 

 was conducted through a vessel containing common salt and 

 zinc filings moistened with dilute hydrochloric acid (as in 

 Bunsen's well-known experiment). The gas issued from a 

 blowpipe-nozzle and burned with an intensely yellow flame, 

 the luminosity of which was not decreased, but rather the 

 reverse, when the flame was plunged into a vessel containing 

 oxygen. 



Inasmuch, therefore, as the decrease in luminosity which a 

 small coal-gas flame suffers when burned in oxygen is due to 

 the presence of an excess of the latter gas, the fact that this 

 decrease does not take place to so marked a degree when the 

 flame is burned in ordinary air is to be traced to the presence 

 of inert nitrogen, which, by diluting the oxygen, diminishes 

 the energy of the oxidation. 



In order to prove the justness of this conclusion, the nitro- 

 gen in a given volume of air was replaced by carbon dioxide ; 

 i. e. a cylinder was filled over water with 1 volume of oxygen 

 and 4 volumes of carbon dioxide ; and, after carefully mixing 

 the gases, a coal-gas flame, burning at the orifice of a small 

 brass tube, was brought into the mixture. The flame conti- 

 nued to burn with a degree of luminosity equal to that which 

 it exhibited in ordinary air ; it follows, therefore, that the fact 

 of dilution alone influences the result, the nature of the diluting 

 gas being unimportant. 



Every indifferent gas, including the products of combustion 

 themselves, must exert a similar influence. When an ordinary 

 flame, issuing from a fine orifice, is burned in oxygen, the 

 luminosity decreases for the reason formerly assigned ; but as 

 soon as the products of combustion (water and carbon dioxide) 

 accumulate sufficiently to dilute the oxygen considerably, the 

 luminosity begins to increase. The flame which had been re- 

 duced to a luminous point becomes enlarged until it presents 

 an appearance similar to that exhibited by it when burning in 

 ordinary air ; this happens at the moment when the oxygen 

 in the vessel is diluted by the products of combustion to the 

 same proportionate extent as it is diluted by nitrogen in the 

 atmosphere. 



If the combustion be continued be}~ond this point, the lumi- 

 nosity again decreases — not as was noticed in the former case, 

 by a great decrease in the size of the flame-mantle, but by 



