642 LECTURE Lr. 



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 

 independently 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 tempera- 

 ture evaporation takes place it is always accompanied by the production of 

 cold ; hence it is usual in warm climates, to em])loy various methods of pro- 

 moting 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 

 freezing point: they then expand agam almost in an equal degree as they 

 are still more cooled ; and provided that they be free from agitation, they may 

 remain liquid at a temperature considerably below the point, at which they 

 usually freeze, and at which their ice always melts. Water may be cooled 

 in this manner to about 10' of Fahrenheit, and if it be then agitated, or 

 especially if a small particle of ice or snow be thrown into it, a certain part 

 of it will instantly congeal, and its temperature will be raised at once 

 to 32°, in consequence of the heat which is always produced or extri- 

 cated in the act of freezing. In most cases, although not in all, the solid 

 occupies more space than the fluid; thus, it is probable that ice, when per- 

 fectly free from air bubbles, is at least one l6th lighter than water at the same 

 temperature. A saturated solution of Glauber's salts, or sulfate of soda,in hot 

 water, may be cooled slowly to the temperature of the atmosphere, when the 

 pressure of the air is excluded, and may be made to crystallize by admitting 

 it suddenly, the liquor becoming at the same tiir^e warm in consequence of 

 the heat which is extricated ; and there is no doubt but that the congelation 

 of water, and perhaps of all other substances, is a crystallization of the same 

 kind. ' 



The expansions of solid bodies appear to be more regular than those of 

 liquids or even of elastic fluids; they vary little at any temperature, although 

 it is said that they do not always take place in their full extent at the instant 



2 



