106 Dr. Karl Heumann's Contributions to 



theless the difference between the two is apparent to the eye, 

 which is able to look at the latter without shrinking, while it 

 is unable to bear the intense light emitted by the former. 

 The delicacy of the eye is therefore very different from that 

 of the photometer. By covering the flame with a shade, 

 arranged so as to permit the rays of light proceeding from a 

 determined part of the flame only to pass outwards, it be- 

 comes possible to measure the luminosity of different parts of 

 the flame by means of the photometer. 



I may at a future time undertake a series of measurements 

 by such a method as this ; meanwhile I should like to put on 

 record the fact that the light emitted by burning a certain 

 petroleum, which is now much used, notwithstanding its high 

 price, on account of the clearness and brilliancy of its flame, 

 is considerably less, as measured by the photometer, than that 

 obtained from ordinary commercial petroleum. 



I propose to use the term intensity of light as applied 

 to the quantity of light emitted by the individual con- 

 stituents of the flame, and the term light-effect as applied to 

 the total effect of the whole of the luminous particles, i. e. of 

 the whole flame. Assuming that the light emitted by the 

 luminous constituents is the same throughout, the " light- 

 effect " would be equal to the product of the " intensity of 

 light " into the volume of the flame-mantle. But inasmuch 

 as this assumption is not proved good, it will not do in con- 

 ducting photometric experiments to pay no attention to the 

 part of the flame from which light is permitted to pass out- 

 wards through the opening of the shade. It is best in all 

 cases to place the shade so that the greatest possible quantity 

 of light may fall upon the diaphragm of the instrument ; the 

 results will then be comparable, so far as the most luminous 

 part of the flame-mantle is concerned. It is just this part of 

 the flame which most materially conditions the character of 

 the whole. 



The most important conclusions arrived at in the second 

 part of this memoir are these : — A cold object brought into a 

 luminous flame causes a suspension of the process of com- 

 bustion in its immediate neighbourhood, and at the same 

 time very materially diminishes the luminosity throughout a 

 considerable space around itself. 



Carbon-containing luminous materials may burn with or 

 without separation of carbon (that is, with or without luminous 

 flames), according as a certain temperature, differing for each 

 material, is or is not maintained. Combustible material which 

 has been diluted with indifferent gases requires to be main- 

 tained at a higher temperature, in order that it shall burn with 

 a luminous flame, than when it is not so diluted. 



