Abstract: When a pure liquid is subjected to an uniform external static field, the external field will alter the interaction among molecules, and decrease the soturated vapour pressure of the liquid. This paper calculated the numerical value of the saturated vaporpres sure of the liquid with the external field and without it .From the calculated results, disscussed one cycle process.The thing interesting people is that the cycle process concerns seemingly the perpetual- motion machine of the second kind.

Key words:  , the saturated vapor pressure, the cycle process, the perpetual-motion machine of the second kind