72 À TEXTBÔÔK ÔF ÔCËANÔGRAPHY 



suitably graduated to read to the fourth décimal ail densities 

 between the above limits. Such an aerometer will give the 

 density of the sea-water at the température at which the reading 

 is made. A correction miist be made to reduce the density to 

 4° C, the température of the maximum density of fresh water. 

 The physical properties of sea-water are those of a dilute 

 solution of salts. Modem théories of chemistry postulate for 

 such solutions similar laws as for gases. When a fluid passes 

 into the gaseous state its particles (molécules) are given off into 

 space by a force known as vapour tension. The phenomena in 

 a solution of sait in water are analogous, the force hère being 

 osmotic pressure. When a solid body is dissolved in water its 

 particles distribute themselves by diffusion, a process which, 

 however, takes place with greater difficulty than the diffusion 

 of gases, and consequently is much slower. 



Solutions of the same freezing-point hâve similar vapour 

 tension and similar osmotic pressure. 



Since solid bodies such as salts hâve a very low vapour 

 tension, it follows that the température of the solution must 

 be lowered below the freezing-point of tlie dissolving substance 

 (in this case for water o° C.) in order that the solution may 

 begin to freeze. Consequently the freezing-point of sea-water 

 varies with the amount of dissolved salts, and the greater the 

 amount of sait the lower the freezing-point. 



On the contrary, the boiling-point of sea-water is higher, 

 for the same atmospheric pressure, than that of fresh water. 

 For the relationships between vapour tension, osmotic 

 pressure, lowering of the freezing-point and raising of the 

 boiling-point, textbooks on physical chemistry should be 

 consulted. There is a direct connection between thèse 

 properties, so that if one be known or estimated the others 

 can be deduced. Of the above four properties, there are 

 considérable practical diffîculties in the correct estimation of 

 three — namely, vapour tension, osmotic pressure, and boiling- 

 point. The estimation of the freezing-point of sea-water is- 

 easier and has been donc with a high degree of accuracy, 



