32 COLD. 



a circular board the seven colours in the proportion in 

 which they are on the spectrum, and by whirling the 

 board round with great velocity, the whole will be so 

 blended together as to appear of a white colour, which 

 will be more or less perfect as the colours are more or 

 less perfectly laid on. 



COLD. 



Effect! of Cold How produced artificially Fatal results from exposure to 

 extreme Cold, &c. 



COLD is that sensation which accompanies a transition 

 of the fine vessels of the human body from an expanded 

 to a contracted state. As heat is said to be caused by a 

 particular motion of the particles of a hot body, so cold 

 is its opposite, or the absence of such motion. Although 

 contraction is the general result of cold, yet some bodies, 

 under peculiar circumstances, expand as they cool : thus 

 iron expands by heat, yet when melted it is found to ex- 

 pand in cooling. Water expands as it is heated, and con- 

 tracts as it cools ; yet just before it begins to freeze, it 

 gradually expands again. The expansive force of water 

 is most astonishing ; it is capable of rending rocks, or of 

 bursting asunder very thick shells of metal. It has been 

 found by experiment, that the expansive power of a 

 spherule of water of one inch in diameter, in freezing, is 

 capable of overcoming a resistance of 27,000lbs. 



Cold may be produced artificially, although it cannot 

 be made to increase itself as heat will do. The greatest 

 degree of cold that has been produced artificially has 

 been eighty degrees below zero or O on Fahrenheit's ther- 

 mometer. Even snow and common salt mixed will sink 

 a thermometer from the freezing point to zero ; and if a 

 little water be poured on a table in a warm room near a 

 fire, and the vessel containing the mixture be put on it, 

 it will become in a very little time completely frozen to 

 the table. 



The effects of extreme cold are very surprising : rivers 

 and lakes become frozen several feet deep : metallic sub- 

 stances blister the skin like red-hot iron : the air, when 



