WATER, ITS PROPERTIES AND FUNCTIONS 227 



is the universal introduction of thermostats for carrying on investigations at a constant 

 temperature. In fact, looking round my own laboratory recently, I noticed that there were 

 five of these adjusted to various constant temperatures. 



Finally, we note that the only other liquid exceeding water in specific heat is 

 liquefied ammonia. 



LATENT HEAT 



Latent heat is the quantity of heat required to change the state of a solid 

 to a liquid, or that of a liquid to a gas, at the same temperature ; or that given 

 out when the reverse change takes place. 



In the case of water, 80 calories are necessary to convert 1 g. of ice at into 

 1 g. of liquid water at the same temperature. This means that as much heat 

 is required for this purpose as to raise the temperature of the resulting 1 g. of 

 liquid from to 80. 



To convert 1 g. of water at 100 to 1 g. of vapour at the same temperature, 

 even more is wanted, viz., 536 calories ; so that to vaporise 1 g. requires as 

 much heat as to raise 536 g. by 1. 



A diphasic system of ice and water is therefore an extremely delicate thermostat. 

 As heat is added or removed, no change of temperature takes place, merely ice is 

 melted or water frozen. In this way, the temperature of large bodies of water 

 never falls below their freezing points, and cannot do so, until the whole mass 

 is frozen through. 



The freezing point of water is not by any means a low one, compared with 

 that of other liquids, and most chemical reactions can take place at this 

 temperature. The latent heat of melting of ice, moreover, is greater than that 

 of any other liquid except ammonia. 



The latent heat of evaporation is more important still in the regulation of 

 temperature. Unlike freezing, evaporation takes place at all temperatures, even 

 below 0. It is naturally greater at higher temperatures, and this fact, in itself, 

 conduces to moderate a rise of temperature when it is already high, while having 

 less effect when the temperature is low. 



After what has already been said, it will not surprise the reader to find that 

 the latent heat of evaporation of water is absolutely the greatest of all substances 

 known, not even excepting ammonia. 



It is to be noted that the large amount of solar heat absorbed in the vaporisa- 

 tion of water from the ocean is recovered again when condensation takes place 

 as rain, and serves not only to warm the cooler places where condensation occurs, 

 but as the source of all the water power of the earth. No other liquid could do 

 this with the same economy of material. 



The importance of evaporation in getting rid of the excess of heat produced in 

 animal metabolism has been referred to above. If the surrounding temperature is 

 the same as that of the organism, no loss can take place by radiation or conduc- 

 tion, so that evaporation is the only means available, but, at the same time, it is 

 the most effective one. 



CONDUCTION OF HEAT 



Here again, water, although a poor conductor compared with metals, takes the 

 highest place among other liquids and even non-metallic solids. The relative 

 values in the following list will illustrate this point : 



Silver - 

 Lead 

 Water - 



1-00 

 0-08 

 0-0125 



Glass - 



Glycerol 



Alcohol 



0-0016 



0-00066 



0-00046 



Thus there is more difference between silver and lead than between lead and water. 

 This fact has its importance in respect of the transference of heat between 

 cells or parts of the same cell where structure prevents convection currents. 



