132 ORIGIN AND NATURE OF LIFE 



atomically, the molecule consisting of only 

 two atoms in each case, and these atoms 

 monads. In each case there is in the molecule 

 one atom of sodium united respectively to 

 one of chlorine, one of bromine, and one 

 of iodine. A clearer case of atomic saturation 

 could not be found ; there is no known way 

 by which an atom of any kind can be intro- 

 duced without splitting up the molecule. When 

 the three crystalline salts separate out from 

 their three respective saturated solutions, 

 it is found, however, that while the sodium 

 chloride molecules have formed themselves 

 into crystals consisting entirely of sodium 

 chloride, the sodium bromide and the sodium 

 iodide have separated in crystals containing 

 water. Analysis of these latter two kinds 

 of crystals, moreover, demonstrates that the 

 bromide and iodide and their respective 

 amounts of water are so beautifully pro- 

 portioned that there are in each case exactly 

 two molecules of water to each molecule of 

 salt. This does not mean that there is the 

 same percentage of water in the sodium 

 bromide as in the sodium iodide crystals, 

 for the sodium iodide molecule is much 

 heavier than the sodium bromide molecule. 

 But when the percentages are worked out on 

 the basis of molecular weights, then it is 



