4. (a) (i) What is the relative humidity of a gas at the temperature at which dew starts to form? Explain your answer. (ii) What would be the R.H. if the temperature were lowered a further 10C? Explain your answer. (b) (i) Why is a Peltier effect cooler used in dew-point measuring instruments? (ii) What is the effect of dust deposits on the mirror in dew-point instruments? What means can be provided to compensate for this effect? (c) What precautions would have to be taken when measuring free water in a liquid? How would the measurement be made?