60S PHILOSOPHICAL TRANSACTIONS. [ANNO I78S. 



straight line is already very well known, it seems unnecessary to give the in- 

 vestigation. 



By a method of reasoning, not very different, may the nature of the curve, 

 which is described by any point of a body moving on an horizontal plane, and 

 affected by friction, be determined. 



XI. Observations and Experiments on the Light of Bodies in a State of Com- 

 bustion. By the Rev. Geo. Cadogan Morgan, of Norwich, p. 190. 



This discussion, says Mr. M. is nothing more than a series of facts, and of 

 conclusions which seem to flow from those facts, and from an attention to the 

 following data. 1 . That light is a body, and like all other bodies subject to the 

 laws of attraction. 2. That light is an heterogeneous body, and that the same 

 attractive power operates with different degrees of force on its different parts. 

 3. That the light which escapes from combustibles when decomposed by heat, or 

 by any other means, was, previous to its escape, a component part of those sub- 

 stances. 



It is an obvious conclusion from these data, that when the attractive force, by 

 which the several rays of light are attached to a body, is weakened, some of those 

 rays will escape sooner than others. Those which are united with the least de- 

 gree of power will escape first, and those which adhere to it most strongly will be 

 the last to quit their basis. We may here have recourse to a familiar fact, which 

 is analogous to this, and will illustrate it. If a mixture, consisting of equal parts 

 of water, of spirits of wine, and of other more fixed bodies, be placed over a fire; 

 the first influence of that heat, to which all the ingredients are alike exposed, will 

 carry off the spirits of wine only : the next will carry off the spirits of wine 

 blended with particles of water : a still greater degree of heat will blend with the 

 vapour which escapes a part of the more fixed bodies, till at length what evapo- 

 rates will be a mixture of all the ingredients which were at first exposed to the 

 fire. In like manner, when the surface of a combustible is in a state of decom- 

 position, those parts which are the least fixed, or which are united to it with the 

 least force, will be separated first. Among these, the indigo rays of light will 

 make the earliest appearance : by increasing the heat we shall mix the violet with 

 the indigo. By increasing it still more we shall add the blue and the green to 

 the mixture, till at length we reach that intensity of heat which will cause all the 

 rays to escape at the same instant, and make the flame of a combustible perfectly 

 white. It is not my present design to show why the most refrangible rays are the 

 first which escape from a burning body, but to enumerate the several facts which 

 seem to show, that such a general law takes place in combustion ; and that the 

 various colours of bodies in this state are uniformly regulated by that decrease of 

 attractive force now described. 



