59 



ture as the beginning, while both the other makers used the freezing 

 point as their zero. 



In the transformation of water into steam a very much greater 

 amount of heat is consumed (about 6% times as much) than by melting 

 ice. This is illustrated to some extent by the comparatively long t^me 

 required to vaporize water after it has reached the boiling point. 

 Since the steam generated has the same temperature as the water from 

 which it has been formed, the length of time required to vaporize the 

 latter compared with that necessary to bring it to the ebullition point 

 indicates to some extent the heat rendered latent. If this latent heat 

 in steam did not exist we should be unable to use boiling water, as at 

 present, for the preparation of our food ; for as soon as it had reached 

 this point it would immediately vaporize to he almost instantly 

 de[)osited again as water on the somewhat cooler materials with which 

 it would come in contact. 



Although the boiling point like the freezing point serves as a 

 standard in the manufacture of thermometers, it is not constant under 

 all circumstances. On the top of mountains it is much lower than at 

 the sea level ; in fact so material is this variation that comparatively 

 small differences in altitude can be determined by it. Thus it may be 

 made to partially serve the purpose of a barometer ; for, like the height 

 of the mercuric column in that instrument, its height is dependent upon 

 the weight of the atmosphere. That by an increased pressure this 

 point is also increased is often seen in the boilers of steam engines ; 

 and that low pressures have the opposite effect is strikingly illustrated 

 by numerous simple experiments. If, for example, a fiask containing 

 some water be heated till it is entirely filled by steam and the residual 

 water, and then tightly corked, the water in it can be made to boil by 

 cooling the flask. The steam being condensed the pressure would be 

 reduced and the vapour developed finding less resistance could pass 

 through the water to the surface and cause what is known as boiling. 

 Water contained in a tube enclosing a partial vacuum can reach this 

 stage when heated by the band. 



The temperature at which water, or rather watery solutions, dis- 

 engage steam, is, in addition to pressure, considerably influenced by the 

 nature and quantity of the materials dissolved. Many gasses reduce 



