372 Dr. Karl Heumann's Contributions to 



shadow ; as the luminous mantle only attains the necessary 

 solidity at some distance above the wick, a space is seen to 

 intervene between the wick and the shadow ; it is only the 

 sides of the lower part of the luminous mantle (through which 

 the sun's rays travel obliquely) that are capable of casting a, 

 shadow ; hence the ^-shaped form of the lower boundary- 

 line of the shadow. Streaks of varying clearness show them- 

 selves throughout the upper parts of the image of the flame ; 

 these are evidently due to the passage of the light through 

 layers of flame of varying densities caused by the differences 

 in temperature of the products of combustion. Fig. 4 repre- 

 sents the shadow thrown by the flame of turpentine ; the soot 

 which appears to the eye in the upper portions of this flame is 

 shown by the shadow to extend continuously almost to the 

 wick. 



It seemed to me necessary to test this new method of examin- 

 ing flames by the shadows which they cast in sunlight, by 

 comparing together as many and as various flames as possible. 



Having proved to myself that the flame of pure hydrogen 

 casts no shadow upon a white screen, but only the image of a 

 few more or less dense streaks (fig. 5), I caused the gas to 

 traverse an apparatus in which it should become laden with, 

 solid material (zinc oxide, magnesia, &c.) before ignition. 

 The flame now in each case cast a most distinct shadow. 

 When finely divided carbon (soot from a turpentine-flame) 

 was thrown into the flame of hydrogen, the flame became 

 strongly luminous, and a shadow at once appeared on the 

 screen. 



As an example of a flame undoubtedly containing solid 

 matter, I employed hydrogen laden with the vapour of chromyl 

 dichloride (Cr0 2 Cl 2 ). During the combustion of this flame 

 chromic oxide is produced ; this oxide is not converted within 

 the flame into a non-solid substance or substances, as is partly 

 the case with the carbon liberated in the flames of hydrocarbo- 

 naceous bodies. Fig. 6 represents the shadow cast by the 

 flame of hydrogen laden with the vapour of chromyl dichlo- 

 ride ; it is very similar to the shadow of the turpentine-flame. 



The observation of the shadow thrown on the screen by a 

 flame serves as a means for determining the action which is 

 taking place within that flame. Spontaneously-inflammable 

 silicon hydride throws a very distinct shadow, resembling that 

 cast by a turpentine-flame. The gas is evidently decomposed 

 with the production of solid silicon, which is again oxidized in 

 the outer portions of the flame to silica, itself also a non-vola- 

 tile solid. 



The flame of hydrogen containing the vapour of osmic acid 



