DECOMPOSITION OP CERTAIN St^BSTANCfiS BY »£AT. ^3 



not sufficiently engaged the attention of chemists: I will 

 therefore endeavour to explain them from the principles I 

 have laid down in a paper on the volatilization of sub- 

 stances, printed in the first volume of the Society of Arcueil. 

 The question to be solved is this: Why, when certain sub* Why? 

 stances of the vegetable or animal kind are distilled, is part 

 decomposed, and part volatilized? Why are they not en- 

 tirely volatilized, or entirely decomposed ? 



The substances, that present to us this kind of altera- Volatilization 

 tion are volatile, and at the same time capable of being de-^j^^^J*^ oyheat 

 composed by heat. Farther, a substance cannot be vola- 

 tilized below the point at which its vapour has a degree of 

 elasticity sufficient to overcome the weight of the atmos- 

 phere, unless this vapour can mix with the air, or some or assisted by 

 other elastic fluid. S^^' 



Now if a substance, that is both volatile and capable of Asubstan«e 

 being decomposed, be subjected to the action of heat, ** JI^fJ^^^T^^^ 

 may happen either that it will be completely volatilized, decomposition, 

 before it experiences a sufficient degree of heat to decom- ^"^ ^^^ *'''"^*'^^' 

 pose it; or that it will be decomposed, before its vapour 

 has acquired a sufficient elasticity, to overcome the pressure 

 of the atmosphere. 



In the first case there is no difficulty: it is that of the First case, 

 distillation of acetic acid, alcohol, ether, volatile oils, &cw 

 As to the substances included in the second, as indigo, thesecoad, 

 oxalic, gallic, and succinic acids, wax, suet, fixed oils, 8cc, 

 they begin to be decomposed, before they are volatilized : 

 but, as their decomposition produces gasses, these gasses Oause of a 

 will cause the volatilization of the part not decomposed, in Pf:*^"*.^ ^**/** 

 the same manner as the air causes that of water below its 

 boiling point. 



Since the gasses that result from the decomposition of a\»hich maybe 

 substance are the cause of its volalilization, and withdraw P'^^®"^®^, 

 it from complete destruction; and as all elastic fluids possess 

 the same properties in this respect; it is easy completely to 

 Tolatilize indigo, several vegetable acids, and many other 

 substances, without their undergoing any alteration. It 

 is sufficient, to keep their temperature a little below that 

 at which they are decomposed, and^o cause a current of 

 some elastic fluid, that has no chemical actioa on diem^ to 

 pass through them. 



These 



