THE COMPOSITION OF WATER, HYDROGEN 135 



it, 2<J lies very much below the ordinary temperature ; that is, that the 

 liquefaction of this yas is only possible at low temperatures, and under 



2) 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 

 2 mm. diameter, water at 15 rises (the height being corrected for the meniscus) 14'8mm., 

 and ether at t to a height 5'35 0'028 t c 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 happen 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. 



These ideas were but little spread until Andrews (1869) explained the matter from 

 another aspect. Starting from gases, he discovered that carbonic anhydride can- 

 not be liqnefied by any degree of compression at temperatures above 81, whilst at 

 lower temperatures it can be liquefied. He called this temperature the critical tem- 

 perature. It is evident that it is the same as the absolute boiling point. We shall after- 

 wards designate it by tc. At low temperatures a gas which is subjected to a pressure 

 greater than its maximum tension (Note 27) is completely transformed into a liquid, 

 which, in evaporating, gives a saturated vapour which possesses 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, which corre- 

 sponds with the critical pressure, which we will designate byjpc and express in atmo- 

 spheres. 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 be 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 



