Resistance and other Properties of Sulphur. 



153 



tion urged is good, and the vapour-pressure of any liquid and the 

 boiling point of that liquid must always be on the same line. I 

 think, however, that it is never absolutely correct, and sometimes it is 

 very far from being true. 



If we suppose a thin film of the liquid to be acted upon with a 

 downward pressure produced by the air and an upward force pro- 

 duced by its vapour- pressure, these two forces, by hypothesis, equal 

 and opposite in direction, can produce no motion in the film. Let the 

 force necessary to produce this motion be called (a) . 



Besides the mere upward motion of the film in a bubble, there is 

 an expanding action which draws out the substance of the bubble, 

 and is resisted by it with force depending upon the nature of the 

 liquid. Let this be called (6). 



Einally, there is a certain amount of force required to burst the 

 particles of the liquid apart when the bubble begins to form. Let 

 this be (c). 



We have, therefore, when bubbles are formed in any liquid, 

 a force equal to the pressure on the surface of the liquid together 

 with a + b + c. 



If the viscosity of the liquid remains the same through the whole 

 range of temperature, a 4- b + c will remain the same, and the line 

 for vapour- pressure and that for boiling point will be parallel, but 

 if instead of this, the liquid changes from being a liquid as mobile 

 as water to a thick viscous body, so stiff that the vessel containing 

 it may be inverted without one drop of the substance being lost, 

 a + b -f- c will change also, and the two lines will not be parallel. 

 The forces a + b + c will be a function of the viscosity. 



To test the truth of this reasoning, I carefully repeated the experi- 

 ments, and found the results to agree with those previously obtained. 

 At the same time, besides noting the pressures at which the bubbles 

 began to form on the surface of the liquid at various temperatures, I 

 observed the pressures at which these bubbles burst, and found that 

 there was a considerable difference. Up to 280° C. it was 4 mm. of 

 mercury, while at 296° C. it fell to 1 mm., 



or A-f B = 4 mm. of mercury up to 280° C. 



A + B = 1 „ 296° C. 



I did not attempt to measure the force C, but I think it probable 

 that it is much greater than A+B, and that the variation of these 

 (A + B + C) in sulphur explains the form of the line found. 



The sulphur molecule is known to undergo various changes, at one 

 temperature containing six atoms, while at another only two enter 

 into its formation. What are the molecular modifications that take 

 place when it cools to a liquid, or when it assumes a semi-fluid state 



VOL. xlvi. m 



