<>N WATF.i; AND ITS CoMI'orNhS 107 



of definite compounds as a class to whose number solutions should (or 

 at least, may) be referred. 



\Vr >a\\ above that copper sulphate loses four- fifths of its water at 

 100 and the remainder at 240. This means that there are two definite 

 compounds of water with the anhydrous salt. Washing soda or car- 

 bonate of sodium, Na 2 CO 3 , separates out as crystals, Na 2 CO 3 ,lCH 2 O, 

 containing G'2 '9 p.c. of water by weight, from its solutions at the 

 ordinary temperature. When a solution of the same salt deposits crystals 

 at a low temperature, about 20, then these crystals contain 71*8 parts 

 of water per 2S-2 parts of anhydrous salt. Further, the crystals are 

 obtained together with ice, and are left behind when it melts. If 

 ordinary soda, with 62 - 9 p.c. of water, be cautiously melted in its own 

 water of crystallisation, there remains a salt, in a solid state, containing 

 only 14-5 p.c. of water, and a liquid is obtained which contains the solu- 

 tion of a salt which separates out crystals at 34, which contain 46 p.c. 

 of water and do not effloresce in air. Lastly, if a supersaturated solu- 

 tion of soda be prepared, then at temperatures below 8 it deposits 

 crystals containing 54'3 p.c. of water. Thus there are known ag many 

 as five compounds of anhydrous soda with water ; and they are dis- 

 similar in their properties and crystalline form, and even in their 

 solubility. "We will mention that the greatest amount of water in the 

 crystals corresponds with a temperature of 20, and the smallest to the 

 highest temperature. There is apparently no relation between the 

 above quantities of water and the salts, but this is only because in each 

 case the amount of water and anhydrous salt was given in percentages, 

 but if it be calculated for one and the same quantity of anhydrous salt, 

 or of water, a great regularity will be observed in the amounts of the 

 component parts in all these compounds. It appears that for 106 parts 

 of anhydrous salt in the crystals separated out at 20 there are 270 

 parts of water ; in the crystals obtained at 15 there are 180 parts of 

 water ; in the crystals obtained from a supersaturated solution 126 parts, 

 in the crystals which separate out at 34, 90 parts, and the crystals with 

 the smallest amount of water, 18 parts. On comparing these quantities 

 of water it may be easily seen that they are in simple proportion to each 

 other, for they are all divisible by 18, and are in the ratio 15 : 10 : 7 : 5 : 1. 

 Naturally, direct experiment, however carefully it be conducted, is 

 hampered with errors, but taking these inevitable errors into con- 

 sideration, it will be seen that for a given quantity of an anhydrous 

 substance there occur, in several of its compounds with water, 

 quantities of water which are in very simple multiple proportion. This 

 is observed in, and is common to, all definite chemical compounds. 

 This rule is called the law of multiple proportions. It was discovered 



