MOLECULF.s AND ATOMS 319 



With the establishment of a true conception of molecules and atoms, 

 chemical formula* became direct expressions, not only of composi- 

 tion, 22 but also of molecular weight or vapour density, and consequently 



because O = ltf, C = 12, Si = 28, &c., are multiples of them. The choice of 

 one or the other multiple quantity for the atomic weight is impossible without a firm and 

 concrete conception of the molecule and atom, and this is only obtained as a consequence 

 of the law of Avogadro-Gerhardt, and therefore the contemporary atomic weights are 

 the results of this law. 



- In order to calculate the percentage amounts of the elements contained in a given 

 compound from its formula, it is necessary to use a very simple proportion sum. Thus, 

 for example, to find the percentage amount of hydrogen in hydrochloric acid we reason 

 as follows: HC1 shows that hydrochloric acid contains 35'5 of chlorine and 1 part of 

 hydrogen. Hence, in :'>C>-r> 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 36' 5 ; therefore, the proportion is as 



1 Of) 



follows # : 1 : : 100 : 36'5 or x = = 2*739. Therefore 100 parts of hydrochloric acid con- 

 36'5 



tain 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 together they should be in simple multiple proportion 

 to each other. Thus, for instance, from the percentage composition of hydrogen peroxide,. 

 5'88 of hydrogen and 94'12 of oxygen, it is easy to find its formula ; it 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 of 1 : 1, which means that in 

 hydrogen peroxide there is one atom of hydrogen to one atom of oxygen. 



The following is a proof of the above practical rule : That to find the ratio of the 

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

 centage amounts by the 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 B, combine 

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

 formula of the substance will be AxBy. From this formula we know that our compound 

 contains xA parts by weight of the first element, and yB of the second. In 100 parts of 



our compound there will be (by proportion) f ' a? . of the first element, and 100 ^ B .., 



xA + yB xA + yB 



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



corresponding combining weights ; we then obtain - . for the first element and 



xA + yB 



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



the ratio of the number of atoms of both 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. 

 Thus the term ' carbon dioxide ' tells more about and expresses CO^ 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 sides of the present nomenclature, Sulphur dioxide, 

 SOo, expresses the same as barium dioxide, BaO^, but sulphurous anhydride indicates 

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

 succeed in embracing both advantages, 



