157 



them instantly to change their condition." Referring to water in 

 illustration, he says that it may be cooled many degrees below 32 

 Fahr., and still retain its liquid state ; yet that if a piece of the same 

 chemical substance ice at a higher temperature be introduced, the 

 cold water freezes and becomes warm. He points out that it is cer- 

 tainly not the change of temperature which causes the freezing ; for 

 the ice introduced is warmer than the water ; and he says he assumes 

 that it is the difference in the condition of cohesion existing on the 

 different sides of the changing particles which sets them free and 

 causes the change. Exemplifying, in another direction, the principle 

 he is propounding, he refers to the fact that water may be exalted to 

 the temperature of 270 Fahr., at the ordinary pressure of the atmo- 

 sphere, and yet remain water; but that the introduction of the 

 smallest particle of air or steam will cause it to explode, and at the 

 same time to fall in temperature. He further alludes to numerous 

 other substances such as acetic acid, sulphur, phosphorus, alcohol, 

 sulphuric acid, ether, and camphine which manifest like phenomena 

 at their freezing- or boiling-points, to those referred to as occurring with 

 the substance of water, ice, and steam ; and he adverts to the ob- 

 served fact that the contact of extraneous substances with the parti- 

 cles of a fluid usually sets these particles free to change their state, in 

 consequence, he says, of the cohesion between them and the fluid 

 being imperfect ; and he instances that glass will permit water to 

 boil in contact with it at 212 Fahr., or by preparation can be made 

 so that water will remain in contact with it at 270 Fahr. without 

 going off into steam ; also that glass can be prepared so that water 

 will remain in contact with it at 22 Fahr. without solidification, 

 but that an ordinary piece of glass will set the water off at once to 

 freeze. 



He afterwards comes to a point in his reasoning which he admits 

 may be considered as an assumption. It is " that many particles in 

 a given state exert a greater sum of their peculiar cohesive force 

 upon a given particle of the like substance in another state than few 

 can do ; and that as a consequence a water particle with ice on one 

 side, and water on the other, is not so apt to become solid as with 

 ice on both sides ; also that a particle of ice at the surface of a mass 

 [of ice] in water is not so apt to remain ice as when, being within the 

 mass there is ice on all sides, temperature remaining the same." 



