Heat and Organization. 169 



when we are told that cooking does not always destroy entozoa 

 inhabiting such a dainty as " measly pork." 



The temperature that will kill all germs of infusorial life is 

 not settled, and it has even been asserted that they are not all 

 destroyed when air is made to pass through tubes filled with 

 red-hot pumice or similar matter. 



Heat increases the mobility of fluids, and hence their pas- 

 sage through small vessels and through the pores of delicate 

 tissues is facilitated by warmth. 



Heat assists the solution of most bodies, and plays an im- 

 portant part in chemical actions. These will form the subject 

 of consideration another time, and they are only mentioned in 

 this place to show they are not forgotten. 



The boiling and freezing points of water are much affected 

 by the presence of other substances in solution. Thus fresh 

 water freezes at 32° F., while sea water freezes at about 28*5°. 

 In like manner pure water boils at the sea level, and at the 

 ordinary barometric pressure, at 212° ; but if saturated with 

 common salt the solution does not boil till it has reached 224°, 

 and if Rochelie salt be substituted for common salt, not till 

 240°. 



The boiling and freezing points of liquids are also regulated 

 by pressure. The greater the pressure the greater being the 

 resistance to any change of condition that demands mobility of 

 particles, or enlargement of bulk. Thus " M. Mousson found 

 that a powerful pressure not only retards the freezing of water, 

 but prevents its complete solidification. In this case the pres- 

 sure opposes the tendency of water to expand on freezing, and 

 thus virtually lowers the point of solidification."* 



With reference to boiling, the rule is that a liquid boils 

 when its tension is equal to the pressure it supports. Now 

 the pressure on a mountain top is considerably less than at its 

 base, and a corresponding lowering of the boiling point is ob- 

 served. On high mountains the direct heat of the sun received 

 by a good absorbing surface, produces a very high tempera- 

 ture with a corresponding dilation of fluids or solids ; but there 

 is an abrupt and violent transition from the heat of direct sun- 

 shine to the cold experienced in the shade. This arises from 

 the rarefaction and dryness of the air at great heights. Sup- 

 pose, then, either plants or animals so situated, that as the 

 sun moves on they are first intensely warm, and then suddenly 

 and violently cooled as his beams are kept off by a wall of 

 rock ; to live under such conditions they must be of a hardy 

 type. 



It often happens that plants will stand a severe cold during 

 the winter, or at night, provided they have enough heat during 

 * G-anot's Physics, by Atkinson, p. 228. 



