[GARVER] THE RANGE OF MOLECULAR ACTION 89 
though the surface-tension exhibited by water at the temperature of 
O°C is only 73-2 dynes per linear c.m. of the film, yet the actual tensile 
strength per unit area of cross-section of the film is about one-fourth 
that of the iron or mild steel used in the shell of steam boilers, although 
its density is not much more than one-eighth as great as that of the iron. 
If in addition, allowance were made for the difference in density and 
we compared the tensile strength of liquid water at O°C with steel con- 
taining the same mass per unit length, the tensile strength of a water 
film would be almost exactly that of the tensile strength of the best 
quality of piano wire. 
In this connection a remark on the importance of rigidity in en- 
chancing tensile strength may not be amiss. Soft iron displays, only in a 
very much less degree than water, the property of elongation, or flowing, 
so that the cross-section diminishes under a stretching force. With 
liquids, this ability to change shape under stress prevents our perceiving 
the actual force necessary to separate two portions of liquid against the 
molecular attractions. When this elongation by diminishing cross- 
section is prevented we may perceive and measure the intensity of the 
molecular attractions. 
Since, as has been shown, the main distinction between surface- 
tension and vapour-pressure is one of sign and phase—the force chang- 
ing sign as the substance changes phase—we should be able to compute, 
approximately at least, the value of € from the surface-tension and the 
general gas equation. In case of liquids at such low temperatures that 
the vapour-density and vapour-pressure are too small to be measured 
accurately, e may be found from the equation 

where m is the molecular weight of the substance as determined from 
its vapour. Applying this equation to mercury at O°C at which tem- 
perature the vapour-pressure and vapour-tension are too small to be 
measured accurately, we find € = 17 X 10~ cm. and the intrinsic 
pressure 1525 atmospheres, assuming the molecular weight to be 198+5 
andiye— 52/7: 
The peculiar relation of surface-tension to vapour-pressure and 
also to the intrinsic pressure of substances may perhaps, be more 
strikingly shown by a slight transformation of the two equations (F) 
and (G). Since mass is the product of volume and density we may 
in each equation substitute the molecular volume, the one liquid, the 
other vapour. They then become 
Sec. III, 1912. 7. 
