MOTIONS OF ODOROUS SUBSTANCES. 3i 



Of thefe three experiments, the two firft mew that camphor Inferences. 

 4&S upon water at a dijhincr, and without contact; the third 

 lenders the manner in which its motions can be- performed *" 

 upon this liquid perceptible to the fenfes. 



It appears to me, that from a comparative view of thefe* 

 facts, we may deduce the following inferences as certain : 



Camphor is moved upon the furface of water by the efTccl 

 Of the emillion of the particles which compofe it ; an emiffion 

 that becomes perceptible to our fenfes by the fmell which it 

 produces, and by the repullions which it exercifes againft fmall 

 bodies floating upon the furface of the water. 



As the effect refulting from thefe different impuifes does not 

 pafs through the centre of gravity of the piece of -camphor, 

 this centre has a progreilive motion, and the body revolves" 

 round it. As the figure of the piece of camphor changes every 

 moment, the motion of its centre of gravity is neither uniform 

 nor rectilinear ; it varies inceitantly, as well as the angular 

 velocity of rotation. As the evaporation takes place princi- 

 pally at the furface of the water, the rotatory motion eitablifhes * 

 itfelf round the axis which is perpendicular to this furface, and' 

 which paffes through the Centre of gravity of the body. 



As, ceteris paribus, the emanation of the particles of the 

 camphor is proportionate to the extent of its furface, and as 

 the furfaees increafe only as the fquares, but the mafles as the' 

 cubes of correfponding dimenfions, the rapidity of the motion 

 of the camphor muft be greater in proportion as its volume is 

 fmaller, and confequently its motion muft Be accelerated in" 

 proportion as it evaporates ; which coincides with experience. 



Having eltabliihed thefe proportions, which in my opinion ' 

 comprehend the true theory of the motions of camphor upon 

 water, let us return to the fecond part of the work of Citizen 

 Benedict Prevoft. 



It includes a great number of experiments, in which we fee Other expen- 

 inodorous fubftance.s produce, upon wet glafs, the fame phe- "? ents w, * b bo " 

 nomena as odorous, oily, or volatile fubftances. 



If we fpread a fmall piece of fine wet linen, of any figure, 

 upon a china plate moiftened with a thin layer of water, the 

 water appears to recede all round it, forming a multitude of 

 jets, and exhibiting the prifmatic colours *. 



* Thefe colours probably refult from the decomposition of the 

 light by the fmall layer of water which furrounds the piece of linen, 

 and which becomes ftill thinner by the retreat of the water. 



If 



