ON WAT Hi; AND ITS COMPOUNDS 53 



vapour and also in liquid water (for its specific heat is greater than 

 that of other liquids) renders it available in both forms for heating 



absorber of heat. This property of water has an important significance in practice and 

 in nature. Water impedes rapid cooling or heating ; it tempers cold and heat. The 

 specific heats of ice and aqueous vapour are much less than that of water ; namely 

 that of ice is 0'504, and of steam 0'48. 



With an irerease in pressure equal to one atmosphere, the compressibility of water is 

 0-000047, of mercury 0-00(KHI:!.VJ, of ether 0'00012 at 0, of alcohol at 13 O'QO.0095. The 

 addition of various substances to water generally simultaneously decreases its com- 

 pressibility and cohesion. The compressibility of other liquids increases with a rise of 

 temperature, but for water it decreases up to 53 and then increases like other liquids. 



The expansion of /rate?- by heat (Note 9) also exhibits many peculiarities which are 

 not found in other liquids. The expansion of water at low temperatures is very small 

 compared with other liquids ; at 4 it reaches even 0, and at 100 it is equal to O'OOOS ; 

 below 4 it is negative i.e., water on cooling then expands, and does not decrease in 

 volume. In passing into a solid state, the specific gravity of water decreases ; at one 

 c.c. of water weighs 0-999888 gram, and one c.c. of ice at the same temperature weighs only 

 0'9175 gram. The ice formed, however, contracts on cooling like the majority of other 

 substances. Thus 100 volumes of ice are produced from 92 volumes of water that is, 

 water expands considerably on freezing, which fact determines a number of natural 

 phenomena. The freezing point of water falls with an increase in pressure (0'007 J per 

 atmosphere), because in freezing water expands (Thomson), whilst with substances which 

 contract in solidifying the melting point rises with an increase in pressure ; thus, for 

 paraffin it is at one atmosphere 46 and at 100 atmospheres 49. 



When liquid water passes into vapour, the cohesion of its particles must be destroyed, 

 as the particles are removed to such a distance from each other that their mutual 

 attraction no longer exhibits any influence. As the cohesion of aqueous particles varies at 

 different temperatures, the quantity of heat which is expended in overcoming this 

 cohesion or the latent heat of evaporation for this reason alone will be different at 

 different temperatures. The quantity of heat which is consumed in the transformation 

 of one part by weight of water, at different temperatures, into vapour was determined by 

 Regnault with great accuracy. His researches showed that one part by weight of water 

 taken at 0, in passing into vapour having a temperature t, consumes 606'5 + 0'305 units 

 of heat, at 50 (521-7, at 100 637'0, at 150 652'2, and at 200 667'5. But this 

 quantity includes also the quantity of heat required for heating the water from to t 

 i.e., besides the latent heat of evaporation, also that heat which is used in heating the water 

 in a liquid state to a temperature t. On deducting this amount of heat, we obtain the 

 latent of evaporation of water as 60t>'5 at 0, 571 at 50, 534 at 100, 494 at 150, and only 

 453 at 200, which shows that the conversion of water at different temperatures into 

 vapour at a constant temperature requires very different quantities of heat. This is 

 chiefly dependent on the difference of the cohesion of water at different temperatures ; 

 the cohesion is greater at low than at high temperatures, and therefore at low tem- 

 peratures a greater quantity of heat is required to overcome the cohesion. On comparing 

 these quantities of heat, it will be observed that they decrease rather uniformly, 

 namely their difference between and 100 is 72, and between 100 and 200 3 is 81 units 

 of heat. From this we may conclude that -this variation will be approximately the same 

 for high temperatures also, and therefore that no heat would be required for the con- 

 version of water into vapour at a temperature of about 400 600 D . At this temperature, 

 water passes into vapour whatever be the pressure (see chap. II. The absolute boiling 

 point of water, according to Dewar, is 370, the critical pressure 196 atmospheres). It 

 must here be remarked that water, in presenting a greater cohesion, requires a larger 

 quantity of heat for its conversion into vapour than other liquids. Thus alcohol consumes 

 208, ether 90, turpentine 70, units of heat in their conversion into vapour. 



The whole amount of heat which is consumed in the conversion of water into vapour 

 is not used in surmounting the cohesion that is, in internal work accomplished in the 



