452 Prof. FORBES on the Vibrations which take place 



ployed copper, brass, iron, and antimony ; on another iron, tin, 

 platinum, and bismuth. I did not find, however, that the addi- 

 tional temperature thus gained facilitated my inquiries, and it 

 was, in the first place, attended with considerable^ practical dif- 

 ficulties. The experiments, however, confirmed a fact which I 

 had previously suspected, and which forms an exception to what 

 may be considered the general law, namely, that the intensity of 

 vibration is proportional to the difference of temperature of the 

 metals ; I found that at 350 iron was far more sluggish in its vi- 

 brations than at 212. I cannot say that I remarked this in the 

 case of copper, brass, or platinum. The fact, however, hardly ad- 

 mits of doubt. At an early period I had been much perplexed 

 with some anomalies in the vibration of iron. When first taken 

 out of a hot open fire, and just cool enough not to melt lead, its 

 action with that metal appeared very unsatisfactory. This effect 

 was so sensible, that I have frequently repeated with success a 

 singularly paradoxical experiment. A bar of iron heated, sup- 

 pose to 212, being placed on a lead block, and the vibrations 

 commenced, if a spirit-lamp was applied to the lower portion of 

 the bar, the vibrations are completely stopped, and may actually 

 be restored by immersing the lead, to which the lamp had been 

 applied, in cold water : these singular effects I have been able to 

 produce several times in succession during one experiment. 



48. The same effects, though less striking, have been pro- 

 duced with zinc instead of iron, which vibrates with consider- 

 able difficulty when the temperature is raised above 212. I 

 have been disposed to consider that every metal has its own 

 most favourable temperature, though on what principle it is not 

 so easy to explain. 



49. It is probable that the softening of the heated metal di- 

 minishes the resiliency of the two bodies when impact takes 

 place. I do not think that it is attributable to the softening of 

 the lead, for I have found that iron is more disposed to vibrate 



