84 SCIENTIFIC RECREATIONS. 



[The difference for a degree depends upon the height, varying between 

 510 and 590 feet, according to the elevation reached. The approximate 

 height of a mountain can be found by multiplying 530 by the number of 

 degrees between the boiling point and 2 1 2. In some very elevated regions 

 travellers have even failed to boil potatoes.] 



The boiling point of liquid may be altered by mixing some substance 

 with it ; and although such a substance as sawdust would not alter the 

 boiling point of water, yet if the foreign matter be dissolved in the liquid it 

 ^.vill alter the boiling point. Even the air dissolved in liquids alters their 

 boiling point, and water freed from air will not boil till it is raised to a 

 temperature much higher than 212 Fahr. Water will boil at a higher 

 temperature in a glass vessel than in metal, because there is a greater 

 attraction between water and glass. 



We said above that an increase of pressure will raise the boiling point 

 of water. Under the pressure of one atmosphere that is, when there is a 

 pressure of 1 5 Ibs. on the square inch water boils at 2 1 2. But under a 

 pressure of two atmospheres, the boiling point rises to 234, and of four atmo- 

 spheres, 294. So we see by increasing the pressure the water may be 

 almost indefinitely heated, and it will not boil. We can understand that in 

 a very deep vessel the layer of water at the bottom has to sustain the 

 pressure of the water in addition to the weight of the atmosphere above it. 

 The pressure of thirty-four feet of water is equal to the atmospheric pressure 

 of I 5 Ibs. on the square inch, and thus at such a distance water must be 

 heated to 234 before it will boil. Professor Bunsen founded his Theory 

 of the Geysers upon this fact, for he maintained that water falling into the 

 earth lost much air, and required with the super-incumbent pressure a very 

 'high temperature to boil it. When it did boil it generated steam so 

 suddenly that it exploded upwards, throwing up vapour and the water with 

 it, as water poured into a very hot basin will do. 



Evaporation may now be considered, and is distinguished from Ebullition 

 iby the production of vapour on the surface of liquids, the latter term signi- 

 fying the formation of vapour in the body of the liquid. Evaporation takes 

 place at all temperatures, and from every liquid surface exposed to the air. 

 We know what we call a " drying wind." The air in fresh layers continually 

 passing over the wet ground, takes up the moisture ; like the east wind, for 

 instance, which has great capabilities of that nature. Damp air can only 

 take up a certain quantity, and when it contains as much water as corre- 

 sponds to the temperature it can take no more, and is "saturated with 

 .moisture " ; then evaporation ceases. Heat is a great cause of evaporation, 

 .and the greater the surface the more rapid the process, and in a vacuum 

 more readily than in atmospheric air. Evaporation is resorted to very 

 commonly to produce coolness ; for instance, the universal fan, by increasing 

 evaporation from a heated skin, generates a feeling of coolness ; and we 

 know the vaporization of ether will freeze into insensibility. When a fluid 

 evaporates we can tell that the heat passes away at the same time, for we 



