82 HEAT OR CALORIC. 



With a mask coated with tin foil, our faces may safely encounter the; 

 blaze of a glass house furnace. lire's Die. 277. 



18. Hot water cools faster in a glass, than in a polished metallic- 

 vessel. 



19. "Radiation of cold. -A thermometer, placed in the focus of 

 a mirror, indicates a decline of temperature, in consequence of a mas? 

 of ice or snow being placed before it, in the situation occupied by the 

 bottle, in the preceding figure. This change of temperature has 

 been ascribed to the radiation of cold, and has been considered a? 

 demonstrating the materiality of that principle. For, since the trans- 

 fer of heat, by radiation, has been adduced as a proof of the exis- 

 tence of a material cause of heat ; it is alleged that the transmission 

 of cold, by the same process, ought to be admitted in evidence, of a 

 material cause of cold."* 



But, it is necessary to suppose only that the heat flows from the 

 thermometer, which is relatively the hotter body, to the ice, which is 

 constantly absorbing the radiant heat of the room and that of the 

 thermometer more than of any other body, because the heat is there 

 concentrated by the mirrors, and thence flows in greater quantities 

 than is true of any other place. f 



III. CONGELATION AND LIQUEFACTION. 



(a.) Dr. Black first proved that fluidity depends on a peculiar com- 

 bination or operation of heat or caloric. 



(b.) The sensible heat of both melting ice and freezing water is 

 at all times and places 32 of jFoAr.J H. 



The water, when first formed by melting, is at 32, and the heat 

 absorbed during liquefaction has merely melted the ice, and has not 

 raised its temperature. If ice is colder than 32% it cannot melt till 

 it attains that temperature, and the sensible heat will neither rise nor 

 fall during the process of melting. 



(c.) The quantity absorbed is 140 A pound of snow at 32 and 

 a pound of water at 172, if quickly mingled, will give the tempera- 

 ture of 32, therefore 140 have been absorbed to melt the ice, and 

 are not discoverable by the senses or by the thermometer.^ 



* Dr. Hare. 



1 Ice at 32,. is a radiant point of heat in an atmosphere of 0, and a freezing mix- 

 ture, e. g. salt and snow producing a cold of 0, would be, relatively, a warm point in 

 a medium of 40 below 0. 



t The freezing and melting points of all bodies are the same lor each particular 

 body, but no two coincide, unless by chance ; c. g. solid mercury melts at 39 

 solid water or ice at-j-32. Most bodies, as the metals, melt without becoming pre- 

 viously soft, but others which are bad conductors, become soft first, as butter and 

 sulphur. 



Several other experiments of Dr. Black, go to prove the same result, namely 

 thai while ice is melting, a quantity of heat enters into it, without raising its tempe- 

 rature, which would raise that of water 140. 



