FREE BOOKS

Author's List




PREV.   NEXT  
|<   159   160   161   162   163   164   165   166   167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183  
184   185   186   187   188   189   190   191   192   193   194   195   >>  
water in the solid state. It must have been observed a very long time ago that water expands when it freezes. Otherwise ice would not float on water; and, what is perhaps more important in your eyes, your water pipes would not burst in winter when the water freezes therein. But although the important fact of the expansion of water on freezing was so long presented to the observation of mankind, it was not till almost exactly the middle of the last century that James Thomson, a gifted Irishman, predicted many important consequences arising from the fact of the expansion of water on becoming solid. The principles lie enunciated are perfectly general, and apply in every case of change of volume attending change of state. We are here only concerned with the case of water and ice. James Thomson, following a train of thought which we cannot here pursue, predicted that owing to the fact of the expansion of water on becoming solid, pressure will lower the melting point of ice or the freezing point of water. Normally, as you are aware, the temperature is 0 deg. C. or 32 deg. F. Thomson said that this would be found to be the freezing point only at atmospheric pressure. He calculated how much it would change with change of pressure. He predicted that the freezing point would fall 0.0075 of a degree Centigrade for each additional atmosphere of pressure applied to the water. Suppose, 267 for instance, our earth possessed an atmosphere so heavy to as exert a thousand times the pressure of the existing atmosphere, then water would not freeze at 0 deg. C., but at -7.5 deg. C. or about 18 deg. F. Again, in vacuo, that is when the pressure has been reduced to the relatively small vapour pressure of the water, the freezing point is above 0 deg. C., _i.e._ at 0.0075 deg. C. In parts of the ocean depths the pressure is much over a thousand atmospheres. Fresh water would remain liquid there at temperatures much below 0 deg. C. It will be evident enough, even to those not possessed of the scientific insight of James Thomson, that some such fact is to be anticipated. It is, however, easy to be wise after the event. It appeals to us in a general way that as water expands on freezing, pressure will tend to resist the turning of it to ice. The water will try to remain liquid in obedience to the pressure. It will, therefore, require a lower temperature to induce it to become ice. James Thomson left his thesis as a prediction. But
PREV.   NEXT  
|<   159   160   161   162   163   164   165   166   167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183  
184   185   186   187   188   189   190   191   192   193   194   195   >>  



Top keywords:

pressure

 

freezing

 

Thomson

 

change

 
atmosphere
 
expansion
 

predicted

 

important

 

liquid

 

remain


freezes

 

general

 

expands

 

thousand

 

possessed

 

temperature

 

require

 
induce
 

vapour

 

reduced


freeze
 
thesis
 

prediction

 

existing

 

appeals

 

evident

 

scientific

 
anticipated
 

insight

 

temperatures


depths

 
obedience
 

atmospheres

 
instance
 

resist

 

turning

 
calculated
 
consequences
 

arising

 

Otherwise


Irishman

 

gifted

 

principles

 

volume

 

attending

 

perfectly

 
enunciated
 

century

 
winter
 

presented