THEORY OF COHESION. 183 



the welding of iron, and to the soldering of all 

 other metals. The adhesive property of wax, 

 gum, sugar, &c. when in a state of fusion by 

 caloric is equally familiar to every one. 



The truth is, that all the internal and external 

 properties of bodies, whether mechanical or che- 

 mical, such as hardness and softness, density and 

 lightness, tenacity and firmness, volatility and 

 combustibility, are determined by their relations 

 to caloric. 



It is well known to spinners and weavers, that 

 the fibres of cotton, wool, flos-silk, and flax, are 

 brittle during cold weather, and that if their 

 rooms are not kept at a proper temperature, 

 their threads often break. On the other hand, 

 some of the most brittle substances in nature, 

 such as glass, may be spun into thread almost 

 as fine as that of the silk- worm, when in a state 

 of fusion by heat ; while butter, lard, and many 

 other substances that are soft and tenacious 

 when warm, become hard and brittle at very low 

 temperatures. The same is true of iron and 

 some other metals, which become brittle like 

 glass or ice during excessively cold weather. 

 The rationale of this will appear from the general 

 fact already established, that the attraction of 

 caloric for bodies augments in proportion as they 

 are deprived of it ; the consequence of which is, 

 that it is more concentrated around their par- 

 ticles, which cohere with a corresponding force, 

 producing great hardness ; but when by a blow, 



