LAWS OF CHEMICAL COMBINATION. NOTATION. 33 



stance crystallizes with water, we usually write the mole- 

 cules of water separately, thus : Xa 2 CO 3 . 10H 2 O, a period 

 separating them ; in place of a period, a plus sign is some- 

 times used. CaSO 4 -f 2H 2 O is the formula, then, of crystal- 

 lized gypsum, the sign + indicating that the connection be- 

 tween the gypsum and the water is not so close as that of 

 the remaining elements. We know this to be a fact, because 

 if crystallized gypsum containing water be heated red-hot, 

 the water is driven off, but the C, the S, and the O 4 are not 

 thus separated. Notice that the 10 prefixed to the H 2 O in 

 the case of sodium carbonate, and the 2 prefixed to the 

 H 2 O in the other formula, multiplies the O as well as the 

 H 2 ; it is the same as if w^e had written H 20 O 10 , or H 4 O 2 , and 

 means that ten and two molecules of water respectively are 

 taken. 



From what we have said about formulae, it is plain that a 

 group of atoms constitutes a molecule, and that the number 

 of atoms in a molecule varies considerably; in HC1 we have 

 two, w r hile in Fe 2 (SO 4 ) 3 , for example, there are seventeen. 

 In the chapters on organic chemistry, you w T ill learn that 

 frequently the organic bodies contain a much larger num- 

 ber of atoms in a molecule. 



30. Further Explanation of Symbols. Since the atoms 

 have definite weights, and elementary bodies unite in fixed 

 proportions by weight, these symbols stand not only for 

 the atoms of the respective elements, but also for definite 

 weights of the elements. The third column in the Table 

 on page 13 gives these weights. S, then, means not only 

 one atom of sulphur, but also 32 parts by weight. Na 

 stands for 23 parts by weight of sodium, and so on. In this 

 light, formulae have a new significance. HC1 means 1 

 part by weight of hydrogen, and one atom or 35.5 parts by 

 weight of chlorine. In sulphuric acid, or H 2 SO 4 , we have 

 the following composition : 



B2 



