108 PRINCIPLES OF CIIK.Al 



by Dal ton, and will be evolved in detail in tiie farther exposition in 

 this work. For the present we will only state that the law of definite 

 composition enables the composition of substances to be expressed by 

 formulae, and the law of multiple proportions permits the application 

 of co-efficients in a weight of whole numbers, in formulae. Thus the 

 formula, Na 2 CO 3 , 10H. 2 O, directly shows that in this crystallo-hydrate 

 there are 180 parts of water to 106 parts by weight of the anhydrous 

 salt, because the formula of soda, Xa.,C0 3 , directly answers to a weight 

 of 106, and the formula of water to 18 parts, by weight, which are hnv 

 taken 10 times. 



Tn the above examples of the combinations of water, we saw the 

 gradually-increasing intensity of the bond between water and a 

 substance with which it forms a homogeneous compound. There is a 

 series of such compounds with water, in which the water is held with 

 very great force, and is only given up at a very high temperature, and 

 sometimes cannot be separated by any degree of heat without the entire 

 decomposition of the substance. In these compounds there is generally 

 . no outward sign whatever of their containing water. A perfectly new 

 substance is formed from an anhydrous substance and water, in which 

 sometimes the properties of neither one nor the other substance are 

 observable. In the majority of cases, a considerable amount of heat is 

 evolved in the formation of such compounds with water. Sometimes 

 the heat evolved is so intense that a red heat is produced and light 

 is emitted. It is hardly to be wondered at, after this, that stable 

 compounds are formed by such a combination. Their decomposition 

 requires great heat ; a large amount of work is necessary to separate 

 them into their component parts. All such compounds are definite, 

 and, generally, completely and clearly definite. The number of such 

 definite compounds with water or hydrates, in the narrow sense of the 

 word, is generally inconsiderable for each anhydrous substance ; in the 

 greater number of cases, there is formed only one such combination of a 

 substance with water, one hydrate, having so great a stability. The 

 water contained in these compounds is often called water of constitution 

 i.e., water which enters into the structure or composition of the given 

 substance. By this it is desired to express, that in other cases tin- 

 molecules of water are as it were separate from the molecules of that 

 substance with which it is combined. It is supposed that in the forma- 

 tion of hydrates this water, even in the smallest particles, forms one 

 complete whole with the anhydrous substance. Many examples of 

 the formation of such hydrates might be cited. The most familiar 

 example in practice is the hydrate of lime, or so-called * slaked ' lime. 

 Lime is prepared by burning limestone, by which the carbonic anhydride 



