PHYSICAL PROPERTIES OF WATER 5 



the hissing which is so often noticeable in jet pumps and injectors, or 

 where water is escaping at high velocities past the restricted area of a 

 valve seat, is due to the reduction of pressure and subsequent liberation 

 of bubbles of air, which occurs under these circumstances. The trouble 

 which is experienced in keeping the air vessels on the delivery side of a 

 pump charged is due to the facility with which water dissolves gases 

 under high pressures. 



Ice Formation. In still water, under normal conditions, when ice is 

 formed this exists as a surface layer. In very cold climates, in addition 

 to existing in the ordinary surface form, ice is found in the form of anchor 

 ice and of frazil. Anchor ice consists of an agglomerated mass of coarse 

 crystals clinging to the bed of the channel and formed there by loss of 

 heat by radiation, the cooling of the bottom thus produced causing ice to 

 form. This can only occur with a clear sky and where the surface ice 

 if. any is transparent. Surface ice will in general effectually prevent its 

 formation. It is usually found in rapid streams, where surface ice cannot 

 form. 



Frazil consists of fine spicular ice crystals floating in the water and 

 formed by slight supercooling below 32 (probably about '001 being 

 sufficient). This adheres to the surface ice and to the anchor ice, and 

 may finally choke up the stream. In engineering work frazil ice is par- 

 ticularly objectionable, as it adheres to the racks and strainers and to the 

 gates of a turbine, and if, due to exposure to air, these are slightly 

 supercooled, freezes into one solid mass. 



In common with all other liquids, water also possesses the properties of 

 Cohesion, Adhesion, and Viscosity. 



ART. 3. COHESION 



is that property of a liquid, or- solid, which enables neighbouring 

 molecules to resist any stress of the nature of a tension. Adhesion is 

 that property which enables it to adhere to a solid body with which it 

 may be in contact. Thus a drop of water exhibits cohesion in its 

 hanging together, and adhesion in virtue of its clinging to a solid body, 

 the force of gravity being overcome both by cohesion and by adhesion. 

 Both phenomena are due to molecular attraction, cohesion between 

 neighbouring molecules of the liquid, and adhesion between those of the 

 solid and liquid. A consideration of the molecular theory of matter 

 indicates that if a is the distance between any two molecules, their 



k 

 mutual attractive force is approximately equal to - 6 ; and it follows that 



