260 Suggestions regarding the Force of Steam. 



not be saturated with moisture, because the vapour is con- 

 tinually attracted and condensed by the glass*. Now if, as 

 De Luc and many others allege, the quantity of vapour con- 

 tained in a given space be independent of the presence of air, 

 might we not suspect that the force of aqueous vapour not 

 mixed with air will also be diminished by its contact with 

 glass, since it is known to be so in the mixed state? But I 

 have no idea of the amount of this diminution, nor how it 

 may vary at different temperatures. However, it is probably 

 so much the greater as the bulk of the vapour in proportion 

 to the containing surface is less; and perhaps every sort of 

 vessel has some effect on the tension or temperature of an in- 

 cluded vapour. May not even the electrical states of the se- 

 veral parts of a complex apparatus have some influence on the 

 elasticity ? 



In such an apparatus as Dr. Ure employed, this supposed 

 effect of glass and of mercurial vapour might be in a great 

 measure obviated by making that end of the tube which 

 contains the vapour to consist of metal. This metallic part 

 should be of such a length as to reach below the oil-bath, 

 and then it may join into a glass tube; but such a joint would 

 be a matter of some nicety. The water again ought to be 

 continued down through the metal part and even a little way 

 into the glass, in order that its contact with the mercury may 

 be seen and kept at the same height in the tube. By this 

 means the mercury may be kept away from the heat, and the 

 pressure of the column of water and vapour resting on the 

 mercury will remain nearly the same, except that it may vai'y 

 a very little as the width of the tube is affected by change of 

 temperature, or by change of strain proceeding from the dif- 



* The peculiar agitation and fluctuation of temperature, which water 

 exhibits while boiling in glass vessels, are probably referable to the same 

 source, being something like the converse of the above process; the force 

 or temperature requisite to form vapour in contact with glass being greater 

 than what, would form it in contact with metal ; for vitreous surfaces are 

 known to attract moisture more strongly than metallic surfaces : and so 

 this vapour on leaving the bottom, being of a temperature above 212°, sud- 

 denly expands and mounts up through the water with greater violence 

 than if only of that temperature. Hence when bits of metal arc thrown 

 in, the vapour rises from them in preference to the glass; and thus the 

 ebullition is rendered more steady. There may, however, be something 

 electrical in this phenomenon. And it may be remarked, in reference to 

 the foregoing explanation, that the temperature at which vapour is formed 

 under water must always, on account of the hydrostatic pressure, be greater 

 than what would form it at the surface. 



The particles of mercpry again are known to attract each other more 

 strongly than they do glass ; but to affirm that this should increase the 

 elasticity of mercurial vapour included in a glass vessel, would be going too 

 far until it were r.ettled by experiment. 



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