ICE, WATER, AKD STEAM 



115 



The boiling point is affected by 

 changes in air pressure. If the air 

 pressure is 30 inches on the barom- 

 eter, the boiling point of pure water 

 will be 100 C. ; if the pressure is 

 greater, the boiling point will be 

 higher ; and if the pressure is lower, 

 water will boil before it is as hot 

 as 100 C. To most of us this makes 

 little difference, but to people who 

 live on the mountains it is some- 

 times a serious matter. In elevated 

 places the air pressure is less than 

 that near the sea level, and the 

 boiling point is lower. On the top 

 of Mont Blanc the boiling point of 

 water is only 84 C. ; at the city of 

 Quito, Ecuador, it is 90 C.; and 

 at many places in our own country 

 the same condition exists (fig. 59). 

 When you boil eggs or potatoes 

 it is the heat and not the boiling 

 that cooks them. If you cannot get 

 water hotter than 90C., for instance, 

 it will take very much longer to cook 

 food materials ; and if the tempera- 

 ture is too low, it may be quite 

 impossible to cook certain foods. 



Note the bubbles rising through 

 This, of course, applies only to those the water and the steam issu- 



foods that are cooked by boiling. in s from the tube - The steam 



When we wish to be sure that foods 



are not heated above 100 during 



cooking, we often cook them in a double boiler or steamer 



to make sure that the only heat they get comes through the 



boiling water so as to prevent burning. 



FIG. 58. How water boils 



is transparent when it is in the 



flask and also when it issues 



from the end of the tube 



