492 LECTURE LI. 



thousandth for each degree : that of water is equal to this at the tem- 

 perature 64, and is greater or less nearly in proportion, to the distance 

 from 39, where it begins, but in high temperatures it varies less, since it 

 is not quite four times as great at the heat of boiling water. The expan- 

 sion of spirit of wine at 70 is six times as great as that of mercury : its 

 utmost variation is much less than that of water, although it is at least 

 twice as great in some parts of the scale as in others. 



It has already been observed that an elevation of temperature consider- 

 ably diminishes the powers of cohesion and of repulsion in solid bodies : 

 the same is also true of liquids ; for the height to which a liquid rises in a 

 capillary tube is diminished somewhat less than -njV^ for each degree of 

 Fahrenheit that the temperature is raised, the force of superficial cohesion 

 being diminished both by the diminution of the immediate actions of the 

 particles, and by that of the distances to which they extend. 



When the temperature of a liquid is so much elevated as to become 

 equal to that of its vapour in a state capable of sustaining the atmo- 

 spherical pressure, or any other pressure which may be substituted for it, 

 a certain portion of the liquid is converted into vapour, and the heat being 

 generally applied at the bottom of the vessel, the vapour rises up in 

 bubbles, and the effect is called boiling. The whole liquid is not converted 

 at once into vapour, because a certain quantity of heat appears to be 

 consumed by the change, and a constant supply of heat is necessary, in 

 order that the operation may be continued. 



It is not, however, only at the boiling point that a fluid begins to be 

 converted into vapour : the pressure of the atmosphere is not sufficient 

 wholly to prevent the detachment of a certain quantity of vapour from its 

 surface, at a temperature which is incapable of supporting it separately in 

 the form of steam in the open air, and it may be thus suspended, when 

 mixed either with common air, or with any other elastic fluid, at the 

 ordinary temperature of the atmosphere. And it appears that the 

 quantity, which is thus suspended, bears very nearly some constant pro- 

 portion to the density of which the steam is capable at the given tem- 

 perature in a separate state, the interposition of the air either not affecting 

 the distance at which the cohesion would take place, or altering it equally 

 in all cases. It seems to be most probable that the density of vapour, 

 suspended in this manner in the atmosphere, is always about twice as 

 great, or at least half as great again, as that of steam existing inde- 

 pendently at the same temperature. There is, perhaps, no liquid absolutely 

 free from a disposition to evaporate : even mercury rises in the vacuum of 

 the barometer, and lines the cavity with small globules ; and it is said that 

 the effect of light is favourable to this slow evaporation. At whatever 

 temperature evaporation takes place it is always accompanied by the 

 production of cold ; hence it is usual in warm climates, to employ various 

 methods of promoting evaporation, in order to lower the temperature 

 of the air, to cool liquids for drinking, or even to procure ice for domestic 

 uses. 



It appears that all aqueous fluids are contracted by cold, until we arrive 

 at a certain point, which is generally about 7 or 8 degrees higher than their 



