22 PRINCIPLES OF CHEMISTRY 



elements enter into the composition of the substance. If we repre- 

 sent the combining weight of the substance A by a and that of the 

 substance B by 6, then the composition of the substance A"B'" will be 

 expressed thus : it contains na parts by weight of the substance A and 

 nib parts by weight of the substance B, and consequently in 100 parts 



of our compound there is contained n percentage parts by weight 



of the substance A and ^ of the substance B. It is evident that 

 na-}- mo 



as a formula shows the relative amounts of all the elements contained 

 in a compound, the actual weights of the elements contained in a given 

 weight of a compound may be calculated from its formula. For example, 

 the formula NaCl of table salt shows (as Na=23 and Cl = 35'5), that 58'5 

 Ibs. of salt contain 23 Ibs. of sodium and 35'5 Ibs. of chlorine, and that 100 

 parts of it contain 39 -3 per cent, of sodium and 60*7 per cent, of chlorine. 



What has been said above clearly limits the province of chemical 

 changes, because from substances of a given kind there can be obtained 

 only such as contain the same elements. But, notwithstanding this 

 primary limitation, the number of possible combinations is infinitely 

 great. Only a comparatively small number of compounds have yet 

 been described or subjected to research, and any one working in this 

 direction may easily discover new compounds which had not before 

 been obtained. It often happens, however, that such newly -discovered 

 compounds were foreseen by chemistry, whose object is the apprehension 

 of that uniformity which rules over the multitude of compound sub- 

 stances, and whose aim is the comprehension of those laws which govern 

 their formation and properties. When once the conception of ele- 

 ments had been established, the most intimate object of chemistry 

 was the determination of the properties of compound substances on the 

 basis of the determination of the quantity and kind of elements of 

 which they are composed ; the investigation of the elements themselves; 

 the determination of what compound substances can be formed from 

 each element and the properties which these compounds show ; and the 

 apprehension of the nature of the connection between the elements in 

 different compounds. An element thus serves as the starting point, 

 and is taken as the primary conception under which all other bodies 

 are embraced. 



When we state that a certain element enters into the composition 

 of a given compound (when we say, for instance, that mercury oxide 

 contains oxygen) we do not mean that it contains oxygen as a gaseous 

 substance, but only desire to express those transformations which 

 mercury oxide is capable of making ; that is, we wish to say that it is 



