FORMULAE, CHEMICAL 469 



the constitution of this acid is suggested by the system introduced by Dr. Frankland. 

 According to his method, sulphuric acid should be thus formulated : S T '0 2 Ho 2 . In 

 this expression, sulphur being the element of highest atomicity is placed at the head 

 of the formula, and written in thick black type, whilst its hexatomic character is in- 

 dicated by the Roman numerals at the upper right-hand side. These six bonds, or 

 units of equivalence, are satisfied partly by the two atoms of diatomic oxygen, and 

 partly by the two atoms of the monatomic radical hydroxyl (HO), which is written by 

 Dr. Frankland Ho. This hydroxyl may be replaced by other radicals ; for example, 

 in zinc sulphate the two atoms of monatomic hydroxyl are replaced by one atom of 

 diatomic zincoxyl, and this salt will be represented by the formula : S yI 2 Zno u . 



The modern doctrine of atomicity has also introduced other forms of constitutional 

 formulae. Thus, many chemists would now express the composition of sulphuric acid 

 in this form : 







H-0-S-O-H 



Such a formula is not intended to indicate the relative positions of the several atoms 

 in the compound, but the lines connecting these atoms are supposed to represent the 

 number of units of equivalency enjoyed by each atom, and the manner in which 

 these bonds of affinity are engaged with those of other atoms. Looking, for example, 

 at the centre of this formula we see that the atom of sulphur has six combining bonds, 

 and how these are engaged with the four atoms of oxygen, each of which has only two 

 such bonds. 



Instead of indicating the atomicity in this way, other chemists prefer to enclose 

 each symbol in a circle from which lines diverge to represent the units of equivalence. 

 Professor Kekule, Dr. Crum Brown, and Dr. Frankland have been foremost among 

 modern chemists in devising these graphic formulae. 



For the purposes of lecture-illustration material expressions may be constructed by 

 means of solid balls representing atoms, and rods representing bonds of union. These 

 concrete systems, called glyptic formulae, were introduced by Dr. Hofmann. 



For reasons given under the article ATOMIC WEIGHTS, it has been considered de- 

 sirable to employ generally throughout this Dictionary the old atomic weights, and a 

 system of dualistic formulae ; but for the convenience of those who are familiar with 

 modern formulae, most of the expressions are also constructed with the modern atomic 

 weights, and these formulae are printed, for the sake of distinction, in thick black type. 



On the modes of determining the empirical and rational formula of substances from 

 the results of their analysis. It now remains to show how the formulae of bodies are 

 determined. There are two kinds of formulae the empirical and rational. An 

 empirical formula merely indicates the simplest ratio existing between the elements 

 present ; a rational formula shows the absolute constitution of an atom or equivalent 

 of any substance. Sometimes the expression rational formula is used in a more 

 extended sense, and then signifies the actual manner in which the elements are 

 arranged in a compound molecule, but this happens so seldom, that we shall in this 

 work understand the term in the sense first given. 



An empirical formula can always be deduced from the mere result of an accurate 

 analysis. A rational formula, on the other hand, demands a knowledge of the atomic 

 weight of the substance. The latter datum can be best determined 1st, by the 

 analysis of a compound with a substance the atomic weight of which is well estab- 

 lished ; 2nd, by determining the density of its vapour. 



Empirical formulas. The percentage composition of a compound having been 

 accurately found, the empirical formula may be deduced from the following rule : 

 Divide the percentage of each constituent by its atomic weight, and reduce the num- 

 ber so obtained to its lowest terms. Suppose, for example, the empirical formula of 

 nitric acid to be required, the percentage being: Nitrogen, 25'9 ; oxygen, 74'1. 



These numbers, divided by their respective atomic weights, give 



To reduce these numbers to their lowest terms, it is merely necessary to divide 

 0-26 by 1-85. The simplest terms being: Nitrogen, 1-00; oxygen, 5'00. Nitric 

 apid consequently consists of one q,ton> of nitrogen, and five of oxygen. 



