134 ORIGIN AND NATURE OF LIFE 



the two cases. If the crystalline salts are 

 heated above the temperature of boiling 

 water, the water of the crystals (water of 

 crystallization) is driven off, and the dry or 

 anhydrous salts are obtained. When these 

 salts are allowed to cool and then mixed 

 with just enough water to form once more 

 the crystals, the mixture as it crystallizes 

 grows quite warm. 



When the chemist measures such quantities 

 of heat, he uses a quantity of the salt equal 

 to the molecular weight of the salt in grammes 

 (the unit of weight of the decimal system), 

 and he expresses the heat developed in units, 

 which he terms calories. The calory is the 

 amount of heat required to heat 100 grammes 

 of water by one degree Centigrade, and is 

 written down as K in the chemist's shorthand. 

 Now when an amount of dried sodium bromide 

 expressing the molecular weight in grammes 

 (i.e., about 110 grammes), is allowed to unite 

 with twice the molecular weight of water in 

 grammes (i.e., 36 grammes), the amount of 

 heat developed is found to be 45 K, or an 

 amount of heat approximately sufficient to 

 heat 45 grammes of water from freezing point 

 to boiling point. 



The figure for the iodide is higher still, 

 standing at 53 K. 



