CALORIC. 23 



very strong tendency to combination, we are not 

 able to procure it in that state. 



Combined caloric is that which is fixed in bodies 

 by affinity or elective attraction, so as to form 

 part of their constitution. By the expression spe- 

 cific caloric of bodies, we understand the respective 

 quantities of caloric requisite for raising bodies of 

 the same weight to an equal degree of temperature. 

 This proportional quantity of caloric is thought to 

 depend upon the distance between the consti- 

 tuent particles of bodies, and their greater or less 

 degrees of cohesion ; and this distance, or rather 

 the space or void resulting from it, is called the ca- 

 pacity of bodies for heat. 



Heat has the property of expanding bodies, or 

 increasing their bulk. This may be observed by 

 fitting a piece of iron to an iron ring so as just to 

 fill it : then if the iron be heated in the fire, it will 

 be found that it has become too large to pass 

 through the ring ; but when cooled, it contracts 

 to the same size as before. Some metals will ex- 

 pand more than others* 



It is supposed that the caloric forces itself be- 

 tween the particles of bodies so as to separate 

 them. In acting thus, it is in direct opposition to 

 the attraction of cohesion, which keeps them 

 together. 



Fluids also expand by heat. Put water into a 

 very small glass tube with a bulb, and apply heat 

 to the bulb ; the water will be seen to expand and 

 fill more of the tube : as it cools, it will contract 

 again. 



Gases also increase in volume, by increase of 

 temperature. Tie the neck of a bladder tight ; 

 when it is almost empty, lay it before the fire ; the 

 included air will expand, and the bladder will swell 



c 4 



