380 Dr. Karl Heumann's Contributions to 



be employed a shadow is obtained. This shadow is most dis- 

 tinct towards the outer edges of the flame, where the thick- 

 ness of luminous matter attains a maximum. 



Fig. 9 represents the shadow thrown by the flame of an 

 ordinary burner. 



Fig. 10 represents the shadow produced by directing the 

 small flame against the screen. Figs. 11 and 12 show the 

 forms of shadows produced by adding an excess of benzol 

 vapour to the gas (coal-gas or hydrogen) issuing from ordi- 

 nary burners. Similar appearances result with the flame 

 of hydrogen rendered luminous by admission of chromyl 

 dichloride. These experiments show that the shadows in- 

 crease in intensity as the thickness of the luminous layer in- 

 creases, or, in other words, that the intensity of the shadow 

 is dependent upon the number of carbon particles which pre- 

 vent the passage of the sun's rays through the flame. 



Hirn blew lycopodium powder into a flame ; before the 

 powder was completely ignited the flame produced a shadow ; 

 after complete ignition, however, no true shadow was obtained. 

 I have carefully repeated this experiment, and find that a 

 flame containing a considerable quantity of lycopodium pow- 

 der produces a marked shadow. Hirn probably used too little 

 of the powder, or placed his screen at too great a distance 

 from the flame. It is of course to be expected that the shadow 

 produced by a flame in which the lycopodium powder is 

 undergoing thorough combustion should be less marked than 

 that formed by a flame which contains unburned lycopodium : 

 in the former case the shadow is due to the presence of sepa- 

 rated carbon only. 



The sole experiment of Hirn which is capable of interpre- 

 tation in terms of his theory only, is that in which the light 

 of burning magnesium wire was passed through the flames of 

 eight petroleum-lamps : the increase in the light-effect of 

 these eight flames was equal to the total light-effect of the 

 magnesium when burned by itself. But Hirn's previous ex- 

 periments showed that the light from two petroleum-lamps 

 loses 39*9 per cent.* when passed through six other similar 

 flames ; it is scarcely possible therefore that the light of burn- 

 ing magnesium could suffer no diminution in luminosity when 

 passed through eight petroleum-flames. Hirn's photometric 

 process is, as his own results show, altogether untrustworthy. 



Hirn says that the flame of the blast-furnace is completely 

 transparent : this may be so. His conclusion, that this flame 

 contains solid matter, but that this solid matter is optically 

 * Calculated according to my method. 



