54 



PRINCIPLES OF CHEMISTRY 



purposes. The chemical reactions which water undergoes, and by 

 means of which it is formed, are so numerous, and so closely allied to 



liquid. A part of this heat is employed in moving the aquj.ms particles; in fact, aqueous 

 vapour at 100 occupies a volume 1,650 times greater than that of water (at the ordinary 

 pressure), consequently a portion of the heat or work is employed in lifting the aqueous 

 particles, in overcoming pressure, or in external work, which may be usefully employed 

 and which is so employed in steam engines. In order to determine this work we will 

 first separately consider all the factors necessary for this calculation, and we will then 

 make a deduction from the comparison of these factors. 



The maximum pressure or tension of aqueous vapour at different temperatures 

 has been determined with great exactitude by many observers. The observations of 

 Regnault in this respect, as on those preceding, deserve special attention from their 

 comprehensiveness and accuracy. The pressure or tension of aqueou* vapour at various 

 temperatures is given in the adjoining table, and is expressed in millimetres of the 

 barometric column having a temperature of 0. 



The table shows the boiling points of water at different pressures. Thus on the 

 summit of Mont Blanc, where the average pressure is about 424 mm., water boils at 

 84*4. In a rarefied atmosphere water boils at even the ordinary temperature, but in 

 evaporating it absorbs heat from the neighbouring parts, and therefore it becomes cold 

 and may even freeze if the pressure does not exceed 4'(5 mm., and especially if the vapour 

 be rapidly absorbed as it is formed. Oil of vitriol, which absorbs the aqueous vapour, is 

 Uried for this purpose. Thus ice may be obtained artificial!}' at the ordinary temperature 

 with the aid of an air-pump. This table of the tension of aqueous vapour also shows the 

 temperature of water contained in a closed boiler if the pressure of the steam formed l>e 

 known. Thus at a pressure of five atmospheres (a pressure of five times the ordinary 

 atmospheric pressure i.e., 5x760 = 3,800 mm.) the temperature of the water would lie 

 152 '. The table also shows the pressure produced on a given surface by steam on issuing 

 from a boiler. Thus steam having a temperature of 152 exerts a pressure of 517 kilos, on a 

 piston whose surface equals 100 sq. c.m., for the pressure of one atmosphere on one 

 sq. c.m. equals 1,033 kilos., and steam at 152 has a pressure of five atmospheres. A> 

 a column of mercury 1 mm. high exerts a pressure of 1'35959 grams on a surface of 

 1 sq. c.m., therefore the pressure of aqueous vapour at corresponds with a pressure of 

 6'25 grams per square centimetre. The pressures for all temperatures may be calculated 

 in a similar way, and it will be found that at 100 it is equal to ].o:;:;--2,s grams. This 

 means that if a cylinder be taken whose sectional area equals 1 sq. c.m.. and if water be 

 poured into it and it be closed by a piston weighing 1,0:!:! grams, thfii on heating it in a 

 vacuum to 100 no steam will be formed, because the steam cannot overcome the pressure 

 of the piston ; and if at 100 534 units of heat be transmitted to each unit of weight of 

 water, {hen the whole of the water will be converted into vapour having the same 

 temperature ; and so also for every other temperature. The question now arises, To 

 what height does the piston rise under these circumstances ; that is, in other words, What 

 is the volume occupied by the steam under a known pressure ? For this we must know 



