150 HEAT. 



If any number of bodies at different temperatures be 

 carried into a room, in a short time the air of the room 

 and the different bodies will all be found to have the 

 same temperature. 



Sir Isaac Newton suggested the following law as to the 

 heat lost, ' that in given small spaces of time the heat 

 lost is always proportional to the heat remaining in the 

 body ; ' by which he calculated several temperatures 

 above the scale of thermometers. This, however, has 

 since been found strictly true only for temperatures 

 below 212; for all above, it is only an approximation. 

 The heat which is lost by a body in cooling is partly con- 

 ducted away by the air, some of it is carried off by cur- 

 rents produced in the air surrounding the body, and 

 some by radiation. 



A hot body will of course heat that portion of the air 

 which immediately surrounds it, which becoming light 

 will rise while another portion of cold air takes 

 its place, and in this way a current will be pro- 

 duced, which much accelerates the cooling of the body. 

 It is evident that this current will be the stronger the 

 higher the temperature of the body. 



If these currents be artificially increased, of course the 

 rate of cooling will be increased. Hence the effect of 

 winds in cooling bodies. The rate of cooling is always 

 proportional to the velocity of the current, or, if the body 

 move, proportional to its velocity. Hence red hot balls 

 which are fired from cannon, should not be fired with 

 great velocities, or at great ranges. 



OF FLUIDITY. 



Dr Black was the first who explained in a satisfactory 

 manner the cause of fluidity. He found that when a 

 solid body was converted into a liquid, a certain quantity 

 of heat combined with it, without sensibly increasing its 

 temperature ; and that this portion of heat is !he cause of 

 fluidity. When a fluid is converted into a solid, a certain 

 quantity of heat leaves it without sensibly diminishing its 

 temperature. 



Experiment. Take a pound of new fallen snow, and 

 add to it a pound of water at 172, the snow will be 

 melted and the whole will be found to have a temperature 



