40 PRINCIPLES OF CHEMISTRY 



CHAPTER I 



ON WATER AND ITS COMPOUNDS 



WATER is found almost everywhere in nature, and in all three physical 

 states. As vapour, water occurs in the atmosphere, and in this form 

 it is distributed over the entire surface of the earth. The vapour of 

 water in condensing, by cooling, forms snow, rain, hail, dew, and fog. 

 One cubic metre (or 1,000,000 cubic centimetres, or 1,000 litres, or 

 35'316 cubic feet) of air can contain at only 4 - 8 grams of water, at 

 20 about 17'0 grams, at 40 about 50*7 grams ; but ordinary air only 

 contains about 60 per cent, of the possible moisture. Air containing 

 less than 40 per cent, of the possible moisture is felt to be dry, and air 

 which contains more than 80 per cent, of the possible moisture is con- 

 sidered as already damp. 1 Water in the liquid state, in falling as rain 



1 In practice, the chemist has to continually deal with gases, and gases are often 

 collected over water; in which case a certain amount of water passes into vapour. 

 and this vapour mingles with the gases. It is therefore most important that he 

 should be able to calculate the amount of water or of moisture in <dr and other gasen. 

 Let us consider the relations in volume and weight which exist in this case. Let us 

 imagine a cylinder standing in a mercury bath, and filled with a dry gas whose volume 

 equals u, temperature t, and pressure or tension li mm. (h millimetres of the column of 

 .mercury at 0). We will introduce water into the cylinder in such a quantity that a -mall 

 part remains in the liquid state, and consequently that the gas will be saturated with 

 aqueous vapour ; the volume of the gas will then increase (if a larger quantity of water be 

 taken some of the gas will be dissolved in it, and the volume may therefore be diminished). 

 We will further suppose that the temperature remains constant after the addition of 

 the water; then the pressure (as the volume increases the mercury in the cylinder 

 falls, consequently the pressure is increased) and the volume is increased. In order to 

 investigate the phenomenon we will artificially increase the pressure, and reduce the 

 volume to the original volume v. Then the pressure or tension will prove greater than 

 h, namely h+f, which means that by the introduction of aqueous vapour the tension 

 of the gas is increased. The researches of Dalton, Gay-Lussac. and Regnatilt showed 

 that this increase is equal to the maximum pressure which is proper to the aqueous 

 vapour at the temperature at which the observation is made. The maximum pressure 

 for all temperatures may be found in the tables made from observations on the tension 

 of aqueous vapour. The quantity/ will be equal to this maximum pressure of aqueous 

 vapour. This may be expressed thus : the maximum tension of aqueous vapour land of 

 all other vapours) saturating a space in a vacuum or in any ,u r as U the same. This 

 rule is known as Dalian's law. Thus we have a volume of dry gas v, under a pressure 

 h, and a volume of moist gas, saturated with vapour, under a pressure // +/. The volume 

 v of the dry gas under a pressure h+f occupies, according to the law of Mariotte, a 



