82 Domestic Science 



which are known as the " critical " temperature and pressure of 

 the liquid, it no longer presents a distinct free surface denoting the 

 boundary between liquid and vapour, and the whole tube is filled 

 with a substance which is in an intermediate state between the 

 liquid and gaseous states, known as the "critical state". Any 

 reduction of either pressure or temperature causes the immediate 

 appearance of both liquid and gas simultaneously in the tube, as 

 shown by the formation of two distinct layers separated by the 

 horizontal free surface of the liquid. 



QUESTIONS 



1. Why is the smell of coal-gas frequently noticeable in all 

 parts of a room in which there is a very small leak in the gas-fittings ? 



2. Carbon dioxide is 22 times as dense as hydrogen. A mixture 

 of equal volumes of these two gases is passed slowly along the stem 

 of a long clay pipe and the gas issuing from the end is collected. 

 What change will have been effected in the composition of the 

 mixture by this treatment ? Give reasons for your answer. 



3. Why do you consider glass to be a solid and treacle to be 

 a liquid ? 



CHAPTER VI 



FLUID PRESSURE 



48. In Chapter in it was stated that the force of 

 gravitational attraction is universal and that the 

 existence of this force is made evident to our senses 

 by the behaviour of a body left unsupported in the 

 air. The force with which the body and the Earth 



