452 Prof. V. B. Lewes. The Cause of Luminosity [Mar 21, 



As has been pointed out by many observers, it is clear that the 

 carbon particles themselves undergo combustion, otherwise they would 

 escape unburnt from the flame, whilst it is manifest that the com- 

 bustion of hydrogen and carbon monoxide, which plays so important 

 a part in the flame, must add its iota to the temperature attained by 

 them. 



Both these sources of temperature, however, would be manifest in 

 the flame itself, and with flames of given size burning from the same 

 description of jet we ought to find that their luminosity is governed 

 by- 



A. The temperature of the flame. 



B. The number of carbon particles in a given area. 



Moreover, we should expect that the higher the temperature of the 

 flame, the whiter would be the light emitted, so that a comparatively 

 low temperature flame, even when rich in carbon particles, would 

 be yellow and lurid as compared with a flame containing the same or 

 a smaller number of particles, but which had a higher temperature. 



It has been pointed out by Professor A. Smithells* that it is 

 erroneous to consider the temperature of a flame as being the tem- 

 perature recorded by thermometric instruments inserted into the 

 flame, as by such devices you only obtain the mean temperature of a 

 considerable area of the flame uncorrected for loss from conduction. 



It is also perfectly well known that in a flame a thick platinum 

 wire may only be heated to redness, whilst a thin wire may even be 

 fused, and this suggests that flame temperatures taken by the Le 

 Chatelier thermo-couple of platinum and platinum-rhodium wires 

 may be totally incorrect. In using this beautiful and convenient 

 device, 1 have found that the length of the wires twisted together 

 made piactically no difference in the recorded temperature, and that 

 one twist was as good as six. 



In all my flame experiments I have made the twist as short as 

 possible, and by always using wires of the same thickness have 

 obtained results which are at any rate comparable if not correct, and 

 in order to find what difference the thickness of-the wires would make, 

 I got Messrs. Johnson and Matthey to draw for me wires of O018, 

 O'Oll, and O003 of an inch diameter, and having calibrated the 

 galvanometer scale for temperature with thermo-couples of the same 

 length of twist made from each of them, obtained the following 

 results with the same portion of a Bunsen flame. 



Wire used. Temperature shown. 



0-018 1617 C. 



O'Oll 1728 



0-003 1865 



* < Phil. Mag.,' 1894, p. 249, 



