110 Prof. Draper 07i the Production of Light 



purity and force was developed, as represented in fig. l,No. 5. 

 Its colour is violet. 



I have made similar experiments on many other flames 

 besides those here mentioned. It is not necessary to relate 

 them in detail, for they give the same results. In every in- 

 stance of combustion in the air, when the flame is bright 

 enough, all the colours arc visible; and when the combustion 

 takes place in oxygen, they are increased in intensity. With 

 hydrogen gas and alcohol, the light is so feeble that the eye 

 cannot catch the terminal rays ; but as soon as the combustion 

 is made in oxygen, the red and the violet both appear, the 

 latter however predominating. Several of these^spectra, both 

 in air and oxygen, are represented in fig. 1. In No. 9, the 

 letters mg and inl indicate a maximum of green and of blue 

 light in the form of bright lines. 



It does not require the use of a prism to satisfy oneself of 

 the change of tint that flames exhibit when the chemical action 

 increases. In reality it is only necessary to contrast the colour 

 of the light emitted in air and oxygen gas. In the latter case 

 rays of a higher refrangibility uniformly arise. 



On the evidence furnished by the foregoing experiments, I 

 regard all flames as consisting of a shell of ignited matter in 

 which combustion is going on with different degrees of rapidity 

 at different depths, being most rapid at the exterior where 

 there is a more perfect contact with the atmosphere, and 

 diminishing inwards. In a horizontal section, the interior 

 space, consisting of unburnt vapour, is black; this is sur- 

 rounded by a ring where the combustion is incipient, and 

 from which red light issues ; then follow orange, yellow, green, 

 blue, indigo, and violet circles in succession, the production 

 of each of these tints being dependent on the rapidity with 

 which chemical action is going forward, that is, on the amount 

 of oxygen present; the tints gradually shading off into one 

 another, and forming, as I have already said, a circular rainbow. 

 An eye placed on the exterior of such a flame, sees all the 

 colours conjointly, and from their general mixture arises the 

 predominant tint. 



An examination of the flame of a candle vertically con^xms 

 this conclusion ; for the red projects on the top of the flame, 

 and the blue towards the bottom. 



From this, which may be regarded as the normal flame, the 

 flame of cyanogen differs. It must consist of as many con- 

 centric shells as the prism separates it into regions of definite 

 refrangibility. The interior part is therefore divided into four 

 red layers, followed by one of orange, one of yellow, seven of 

 green, &c. There are two great inactive spaces towards the 



