Note 29 — Chapter II · THE COMPOSITION OF WATER, HYDROGEN
George Kamensky translation
Cagniard de Latour, on heating ether in a closed tube to about 190°, observed that at this temperature the liquid is transformed into vapour occupying the original volume—that is, having the same density as the liquid. The further investigations made by Drion and myself showed that every liquid has such an absolute boiling point, above which it cannot exist as a liquid and is transformed into a dense gas. In order to grasp the true signification of this absolute boiling temperature, it must be remembered that the liquid state is characterised by a cohesion of its particles which does not exist in vapours and gases. The cohesion of liquids is expressed in their capillary phenomena (the breaks in a column of liquid, drop formation, and rise in capillary tubes, &c.), and the product of the density of a liquid into the height to which it rises in a capillary tube (of a definite diameter) may serve as the measure of the magnitude of cohesion. Thus, in a tube of 1 mm. diameter, water at 15° rises (the height being corrected for the meniscus) 14·8 mm., and ether at t° to a height 5·35 - 0·028t° mm. The cohesion of a liquid is lessened by heating, and therefore the capillary heights are also diminished. It has been shown by experiment that this decrement is proportional to the temperature, and hence by the aid of capillary observations we are able to form an idea that at a certain rise of temperature the cohesion may become = 0. For ether, according to the above formula, this would occur at 191°. If the cohesion disappear from a liquid it becomes a gas, for cohesion is the only point of difference between these two states. A liquid in evaporating and overcoming the force of cohesion absorbs heat. Therefore, the absolute boiling point was defined by me (1861) as that temperature at which (a) a liquid cannot exist as a liquid, but forms a gas which cannot pass into a liquid state under any pressure whatever; (b) cohesion = 0; and (c) the latent heat of evaporation = 0.
This definition was but little known until Andrews (1869) explained the matter from another aspect. Starting from gases, he discovered that carbonic anhydride cannot be liquefied by any degree of compression at temperatures above 31°, whilst at lower temperatures it can be liquefied. He called this temperature the critical temperature. It is evident that it is the same as the absolute boiling point. We shall afterwards designate it by tc. At low temperatures a gas which is subjected to a pressure greater than its maximum tension (Note 27) is transformed into a liquid, which, in evaporating, gives a saturated vapour possessing this maximum tension; whilst at temperatures above tc the pressure to which the gas is subjected may increase indefinitely. However, under these conditions the volume of the gas does not change indefinitely but approaches a definite limit (see Note 28)—that is, it resembles in this respect a liquid or a solid which is altered but little in volume by pressure. The volume which a liquid or gas occupies at tc is termed the critical volume, and corresponds with the critical pressure, which we will designate by pc and express in atmospheres. It is evident from what has been said that the discrepancies from Mariotte and Boyle's law, the absolute boiling point, the density in liquid and compressed gaseous states, and the properties of liquids, must all he intimately connected together. We will consider these relations in one of the following notes. At present we will supplement the above observations by the values of tc and pc for certain liquids and gases which have been investigated in this respect—
tc pc tc pc
N2 - 146° 33 H2S + 108° 92
CO - 140° 39 CH2N2 + 124° 62
O2 - 119° 50 NH3 + 131° 114
CH4 - 100° 50 CH3Cl + 141° 73
NO - 93° 71 SO2 + 155° 79
C2H4 + 10° 51 C5H10 + 192° 34
CO2 + 32° 77 C4H10O + 193° 40
N2O + 53° 75 CHCl3 + 268° 55
C2H2 + 37° 68 CS2 + 278° 78
HCl + 52° 86 C6H6 + 292° 60
H2O + 365° 200 C6H5F + 287° 45
CH3OH + 240° 79 C6H5Cl + 360° 45
C2H5OH + 243° 63 C6H5Br + 397° 45
CH3COOH + 322° 57 C6H5I + 448° 45
Young and Guy (1891) showed that tc and pc clearly depend upon the composition and molecular weight.