THE COMPOSITION OF WATER, HYDROGEN 133 



perature, does not pass into a liquid state under a very consider- 

 able pressure,- 7 but is compressed into a lesser volume than would 



Regnault, about 1850, showed that Gay-Lussac's law is not entirely correct, and that 

 different gases, and also one and the same gas at different pressures, have not quite the 

 same coefficients of expansion. Thus the expansion of air between and 100 is 0'367 

 under the ordinary pressure of one atmosphere, and at three atmospheres it is 0'371, the 

 expansion of hydrogen is 0'366, and of carbonic anhydride 0'37. Regnault, however, did 

 not directly determine the change of volume between the and 100, but measured the 

 variation of tension with the change of temperature ; but as gases do not entirely follow 

 Mariotte's law, therefore the change of volume cannot be directly judged by the variation 

 of tension. The investigations carried on by myself and Kayander, about 1870, showed 

 the direct variation of volume on heating from O 3 to 100. These investigations confirmed 

 Regnault's conclusion that Gay-Lussac's law is not entirely correct, and further showed 

 (1) that the expansion per volume from to 100 J under a pressure of one atmosphere, 

 for air -0-368, for hydrogen = 0'367, for carbonic anhydride = 0'373, for hydrogen bromide 

 = 0'386, &c. ; (2) that for gases which are more compressible than should follow 

 from Mariotte's law the expansion by heat increases with the pressure for example, 

 for air at a pressure of three and a half atmospheres, it equals 0'371, for carbonic 

 anhydride at one atmosphere it equals 0'373, at three atmospheres 0'389, and at eight 

 .atmospheres 0'413 ; (3) that for gases which are less compressible than should follow 

 from Mariotte's law, the expansion by heat decreases with an increase of pressure' 

 for example, for hydrogen at one atmosphere 0'367, at eight atmospheres 0'369, for air at 

 a quarter atmosphere 0"370, at one atmosphere 0'368 ; and hydrogen like air (and all 

 gases) is less compressed at low pressures than should follow from Mariotte's law (air 

 at higher pressures than the atmospheric pressure gives a contrary result), as investiga- 

 tions made by myself, aided by Kirpicheff and Hemilian, showed. Hence, hydrogen, 

 starting from zero to the highest pressures, exhibits a gradually, although only slightly, 

 varying coefficient of expansion, whilst for air and other gases at the atmospheric and 

 higher pressures, the coefficient of expansion increases with the increase of pressure, so 

 long as their compressibility is greater than should follow from Mariotte's law. But 

 when at considerable pressures, this kind of discrepancy passes into the normal (see Note 

 25), then the coefficient of expansion of all gases decreases with an increase of pressure, 

 as is seen from the researches of Amagat. The difference between the two coefficients 

 of expansion, for a constant pressure and for a constant volume, is explained by these 

 relations. Thus, for example, for air at a pressure of one atmosphere the true coefficient 

 of expansion (the volume varying at constant pressure) = 0'00368 (according to Mende- 

 leeff and Kayander) and the variation of tension (at a constant volume, according to 

 Regnault) =0'00367. 



27 Permanent gases are such as cannot be liquefied by an increase of pressure alone. 

 With a rise of temperature, all gases and vapours become permanent gases. As we shall 

 afterwards learn, carbonic anhydride becomes a permanent gas at temperatures above 

 31, and at lower temperatures it has a maximum tension, and may be liquefied by 

 pressure alone. 



The liquefaction of gases, accomplished by Faraday (see Ammonia) and others, in 

 the first half of this century, showed that a number of substances are capable, like water, 

 of taking all three physical states, and that' there is no essential difference between 

 vapours and gases, the only distinction being that the boiling points (or the temperature 

 at which the tension =760 mm.) of liquids lie above the ordinary temperature, and those 

 of liquefied gases below, and consequently a gas is a superheated vapour, or vapour 

 heated above the boiling point, or removed from saturation, rarefied, having a lower 

 tension than that maximum which is proper to a given temperature and substance. We 

 will here cite, as we did for water (p. 54), the maximum tensions of certain liquids and 

 gases at various temperatures, because they may be taken advantage of for obtaining 

 constant temperatures by changing the pressure at which boiling or the formation of 



