[ 416 ] 



LXXV. On the Elaiticity of Fluids. 



To Mr. Tilloch. 



SjRj — 1 HE particles of elastic fluids are generally supposed to 

 repel each other : and from the manner in which the first prin- 

 ciples of pneumatics are investigated, it would appear that this 

 property is necessary to the existence of an elastic fluid. If we 

 admit that it is.^ we must also admit that the same particles may 

 at one time attract, and at another time repel each other. 

 For the particles of water attract each other, and when water 

 is converted into steam its properties are not altered ; but steam 

 is an elastic fluid, therefore the particles must have been sepa- 

 rated beyond their sphere of attraction, and must have got within 

 a sphere of repulsion. How distance alone can change the 

 properties of matter we are at a loss to conceive. 



Let us endeavour to explain the phaenomena on other princi- 

 ples. Heat and water combine with each other in a constant 

 proportion under the same atmospheric pressure; but in this 

 stale it can hardly be called an elastic fluid, because on being 

 pressed the heat is expelled, and part of it becomes water ; but 

 if it can instantly regain the quantity of heat expelled, it is 

 elastic ; and this elasticity is owing to the strong tendency of 

 heat and water to conibine, when a certain proportion of heat 

 is present : this proportion varies as the pressure. When steam 

 IS compressed, heat is given out ; and it is not compressed into 

 a less space, as the advocates for the repulsive system suppose it 

 to be. 



In your paper on Elasticity (Phil. Mag. vol. xxii.) you say 

 that attraction and heat are the causes of elasticity: but all the 

 phssnomena may be exhibited without the assistance of heat : 

 for instance, immerse a sponge in water, press it and remove 

 the pressure, it is elastic, and attraction and form are tlie 

 causes. 



I have not, hitherto, seen any experiments to prove the ex- 

 istence of repulsion, that do not admit of a satisfactory explana- 

 tion from attraction only, and we never ought to admit of more 

 causes than are sufficient to account for the eflTccts. 



1 was led to make the above observations, from seeing a paper 

 On Chemical Composition (Thomson's Annals for November, 

 p. 376), where the doctrine of repulsion is brought forward to 

 explain a part of that theory. 



I am, &c. 



Beiitinck-strcet, Dec. 7, Ifili T. T. 



LXXVI. General 



