and on Dissociation. 457 



the disengagement of heat produced during combination, and 

 arising from this solidification, might be considerable without 

 any change in the volume, or without any sensible variation in 

 the distance of the compound. 



Pursuing the comparisons which I have made between affinity 

 and the force of cohesion, and supposing (which is in accordance 

 with prevalent ideas, especially in organic chemistry*) that each 

 molecule of hydrogen and chlorine is double, we see in fact that 

 everything I have said about compound bodies and their disso- 

 ciation applies to hydrogen and to chlorine. They are in the same 

 physical state as water dissociated at 1000° ; and at the moment 

 at which they combine, by rendering it sensible they lose a 

 quantity of heat corresponding to that which the dissociated 

 vapour of water ought to give up, in order to revert to its ordi- 

 nary condition of stability. From this point of view I can con- 

 sider hydrogen and chlorine themselves as bodies dissociated at 

 the ordinary temperature like water heated above 1000°. 



Continuing this comparison, and anticipating a research with 

 which I am occupied relative to the temperature of decomposi- 

 tion of bodies which disengage a gaseous element, I think I am 

 authorized in assimilating these phenomena to those of the 

 ebullition of liquids, or the vaporization of non-liquifiable solids. 

 Water deprived of air boils with great difficulty in glass, and, 

 according to M. Donny, can even remain in the liquid state up 

 to 140° at the ordinary pressure. But if a fragment of pla- 

 tinum, or some metallic powder be thrown in, the vapour is 

 disengaged with extreme violence. It is a phenomenon of 

 the same kind which is produced when oxide of copper or 

 of manganese is placed in fusing chlorate of potass. The 

 boiling-point is lowered, if I may so express myself, or, in lessen- 

 ing by a mechanical means the stability of the compound, the 

 decomposition of the chlorate of potass may be likened to the 

 destruction of the cohesion of water heated above 212° by a 

 platinum point. This I believe to be the simplest explanation 

 of the singular experiment which we every day perform in our 

 laboratories in preparing oxygen from a mixture of oxide of man- 

 ganese and chlorate of potass. 



In a subsequent communication I shall describe the experi- 

 ments which I have made with a view of defining the influence 

 of velocity in chemical actions, and shall oppose the hypothesis 

 which Berthollet has introduced into science under the name of 

 action of mass. 



M. Thenard, by a most ingenious and conclusive experiment, 



* To make the hypothesis 9=16, H=l, Cl=35'5, is exactly the same 

 as dividing the molecule of hydrogen and chlorine so that 0=8, H=i, 

 C1=17'7j and referring these equivalents to one volume instead of to two, 

 as is proposed at present. 



Phil. Mag. S. 4. Vol. 20. No. 135. Dec. 1860. 2 H 



