464 



[March 5, 



The form was not altered in the least, though the heat was raised 

 in the last instance to the melting-point, as shown by the lower part 

 of the cylinder beginning to melt. 



Experiment 13. A hollow zinc cylinder was heated and cooled by 

 half-immersion fifty times. 



It was heated in a wood furnace, the degree of heat to which it 

 was brought being regulated by the melting of a piece of tin which 

 was conveyed at the same time with it into the furnace. Several 

 experiments with pieces of tin and zinc had been previously made, 

 by means of which it was ascertained that in the same temperature 

 tin melted in two-sevenths of the time requisite to melt zinc ; hence 

 when the zinc cylinder and piece of tin were placed in the furnace 

 together, the time occupied by the tin in reaching its melting-point 

 was carefully noted, and the cylinder was left in the furnace as long 

 again as the time thus observed ; by this means it was brought very 

 nearly to its melting-point without incurring any danger of its actually 

 melting. The last five times, however, it was allowed to remain a 

 little longer in the flame; and the melting upon the top was re- 

 tarded the last four times by placing a piece of iron upon it, which 

 conducted heat from that part, allowing it to remain half a minute 

 longer in the furnace. 



The effect obtained was the same as that produced upon the brass 

 cylinder (Exp. 1 0), or the opposite of what took place with iron ; an 



Fig. 17. 



