ON AVATKK AND ITS COMPOUNDS 103 



, <-liaii!_;<' ft its properties. In certain cases the water of crys- 

 tallisation is only given oft' when the solid form of the substance is 

 destroyed : when the crystals melt on heating. The crystals are then 

 said to ma/t in their water of crystallisation. Further, after the separa- 

 tion of the water, a solid substance remains behind, so that by further 

 heating it acquires a solid form. This is seen most clearly in crystals 

 of sugar of lead or lead acetate, which melt in their water of crystalli- 

 sation at a temperature of 56*25, and in so doing begin to lose water. 

 On reaching a temperature of 100 the sugar of lead solidifies, having 

 lost all its water ; and then at a temperature of 280 the anhydrous and 

 solidified salt again melts. Sodium acetate (C 2 H 3 Na0 2 ,3H.,O) melts 

 at .">8 (but resolidifies only on contact with a crystal, otherwise it may 

 remain liquid even at ; as the temperature does not change during 

 solidification, the melted salt can be used for obtaining a constant 

 temperature of 58). According to Jeannel, the latent heat of fusion is 

 about 28 calories, and, according to Pickering, the heat of solution is 35 

 calories. When melted, this salt boils at 123 that is, the tension of 

 the aqueous vapour given off then equals the atmospheric pressure. 



It is most important to recognise in respect to the water of crys- 

 tallisation that its ratio to the quantity of the substance with which it 

 is combined is always a constant quantity. However often we may 

 prepare copper sulphate, we shall always find 36*14 p.c. of water in its 

 crystals, and these crystals always lose four-fifths of their water at 

 100, and one-fifth of the whole amount of the water contained remains 

 in the crystals at 100, and is only expelled from them at a temperature 

 of about 240. The determination of the amount of water of crystal- 

 lisation is easily made if a weighed quantity of crystals is dried in an 

 air or other bath. What has been said about crystals of copper sulphate 

 refers also to crystals of every other substance which contain water of 

 crystallisation. It is impossible to here increase either the relative 

 proportion of the salt or of the water, without changing the homo- 

 geneity of the substance. If once a portion of the water be lost for 

 instance, if once efflorescence takes place a mixture is obtained, and 

 not a homogeneous substance, namely a mixture of a substance deprived 

 of water with a substance which has not yet lost water i.e., decom- 

 position has already commenced. This constant ratio is an example of 

 the fact that in chemical compounds the quantity of the component 

 parts is quite definite ; that is, it is an example of the so-called definite 

 (lumical compounds. They may be distinguished from solutions, and 

 from all other so-called indefinite chemical compounds, in that at least 

 one, and sometimes both, of the component parts may be added in a 

 large quantity to an indefinite chemical compound without destroying 



