266 ESSAY 



that old pans, which permit this in a less de- 

 gree, are less fit for the making of ice. But the 

 argument, which is hence derived by them, is 

 completely refuted by a fact related by Mr. 

 Williams himself; for he says, that the pans are 

 greased before they are used, to prevent the 

 adhesion of the ice to their sides ; since, if this 

 purpose be answered, the water can never be in 

 contact with the pans, and therefore can never 

 pass through them. 



The real reason of the less fitness of old pans 

 for the making of ice is perhaps the following. 

 The production of the cold, which occurs in 

 this process, must take place in the water; 

 since neither the straw upon which the pans 

 are placed, nor the air above them, was ever 

 found by Mr. Williams of so low a temperature 

 as 32. l ' Whatever, therefore, obstructs the 

 passage of heat from the straw to the water, 

 must favour the freezing of the latter. But 

 this will be less effectually done by an old than 

 by a new pan, as the density of the former is 

 greater, from the grease forced into it by rub- 

 bing, and from the slime and sand that will 

 enter with the water into its pores, when these 

 are not entirely closed by the grease ; which 

 must often happen, as the smearing is performed 

 only once in three or four days. The difference, 

 however, in effect betwixt old and new pans 



