326 PRINCIPLES OF CHEMISTRY 



With the establishment of a true conception of molecules and 

 atoms, chemical formulae became direct expressions, not only of com- 

 position, 22 but also of molecular weight or vapour density, and conse- 

 quently of a series of fundamental chemical and physical data, inasmuch 

 as a number of the properties of substances are dependent on their 



w The percentage amounts of the elements contained in a given compound may be 

 calculated from its formula by a simple proportion. Thus, for example, to find the 

 percentage amount of hydrogen in hydrochloric acid we reason as follows : HC1 shows 

 that hydrochloric acid contains 85'5 of chlorine and 1 part of hydrogen. Hence, in 86'5 

 parts of hydrochloric acid there is 1 part by weight of hydrogen, consequently 100 parts 

 by weight of hydrochloric acid will contain as many more units of hydrogen as 100 is 



greater than 86'5; therefore, the proportion is as follows a; 1 " 100 86-5 or x = 2 



2'739. Therefore 100 parts of hydrochloric acid contain 2*739 parts of hydrogen. In 

 general, when it is required to transfer a formula into its percentage composition, we 

 must replace the symbols by their corresponding atomic weights and find their sum, and 

 knowing the amount by weight of a given element in it, it is easy by proportion to find 

 the amount of this element in 100 or any other quantity of parts by weight. If, on the 

 contrary, it be required to find the formula from a given percentage composition, we must 

 proceed as follows: Divide the percentage amount of each element entering into the 

 composition of a substance by its atomic weight, and compare the figures thus obtained 

 they should be in simple multiple proportion to each other. Thus, for instance, 

 from the percentage composition of hydrogen, peroxide. 6'88 of hydrogen and 94'12 of 

 oxygen, it is easy to find its formula; at is only necessary to divide the amount of 

 hydrogen by unity and the amount of oxygen by 16. The numbers 5 - 88 and 5*88 are 

 thus obtained, which are in the ratio 1 : 1, which means that in hydrogen peroxfde there 

 is one atom of hydrogen to one atom of oxygen. 



The following is a proof of the practical rule given above that to find the ratio of the 

 number of atoms from the percentage composition, it is necessary to divide the per- 

 centage amounts by tjie atomic weights of the corresponding substances, and to find 

 the ratio which these numbers bear to each other. Let us suppose that two radicles 

 (simple or compound), whose symbols and combining weights are A and 6, combine 

 together, forming a compound composed of x atoms of A and y atoms of B. The 

 formula of the substance will be Ax"By, From this formula we know that our compound 

 contains xA. parts by weight of the first element, and z/B of the second. In 100 parts of 



our compound there will be (by proportion) ^ of the first element, and 



' a-A + i/B a;A + 



of the second. Let us divide these quantities, expressing the percentage amounts by the 

 corresponding combining weights; we then obtain _J? for the first element and 



.1 . .*/ . for the second element. And these numbers are in the ratio x : y that is, in 



the ratio of the number of atoms of the two substances. 



It may be further observed that even the very language or nomenclature of chemistry 

 acquires a particular clearness and conciseness by means of the conception of molecules, 

 because then the names of substances may directly indicate their composition. Thud 

 the term ' carbon dioxide ' tells more about and expresses CO 2 better than carbonic acid 

 gas, or even carbonic anhydride. Such nomenclature is already employed by many. But 

 expressing the composition without an indication or even hint as to the properties, would 

 be neglecting the advantageous side of the present moinenclature. Sulphur dioxide, 

 60 2 , expresses the same as barium dioxide, BaO 2 , but sulphurous anhydride indicates 

 the acid properties of SO 2 . Probably in time one harmonious chemical language will 

 succeed in embracing both advantages. 



