418 PRINCIPLES OF CHEMISTRY 



to foretell, if not the properties (for example, the acidity or basicity), 

 at any rate the composition, 1 of some of their compounds. This forms 

 the substance of the conception of the valency or atomicity of the elements. 

 Hydrogen is taken as the representative of the univalent elements, 

 giving compounds, RH, R(OH), R 2 O, RC1, R 3 N, R 4 C, &c. Oxygen, 

 in that form in which it gives water, is the representative of the 



1 But it is impossible to foretell all the compounds formed by an element from its 

 atomicity or valency, because the atomicity of the elements is, variable, and furthermore 

 this variability' is not identical for different elements. In CO 2 , COX 3 , CH^, and the 

 multitude of carbon compounds corresponding with them, the C is quadrivalent, but in 

 CO either the carbon must be taken as bivalent or the atomicity of oxygen be accounted 

 as variable. Moreover, carbon is an example of an element which preserves its atomi- 

 city to a greater degree than most of the other elements. Nitrogen in NH 3 , NH 2 (OH), 

 N 2 O 3 , and even in CNH, must be considered as trivalent, but in NH 4 C1, NO 2 (OH), and 

 in all their corresponding compounds it is necessarily pentavalent. In N 2 O, if the- 

 atomicity of oxygen = 2, nitrogen 'has an uneven atomicity (1, 8, 5), whilst in NO it is 

 bivalent. If sulphur be bivalent, like oxygen, in many of its compounds (for example, 

 H 2 S, SC1 2 , KHS, &c.), then it could not be foreseen from this- that it would form SO 2 , 

 SO 3 , SC1 4 , SOC1 2 , and a series of similar compounds in which its atomicity must be 

 acknowledged as greater than 2. Thus SO 2 ," Sulphurous anhydride, has many 

 points in common with CO 2 , and if carbon , be quadrivalent then the S in SO 2 is 

 quadrivalent. Therefore the principle of atomicity (valency) of the elements cannot be 

 considered established as the basis for the study of the elements, although it gives an easy 

 method of grasping many analogies, I consider the four following as the chief obstacles 

 to acknowledging the atomicity of the elements as a primary conception for the con- 

 sideration of the properties of the elements : 1. Such univalent elements as H, Cl, <fcc., 

 appear in a free state as molecules H 2 , C1 2 , &c., and are consequently like the univalent 

 radicles CH 3 , OH, CO 2 H, &c., which, as might be expected, appear as C 2 H 6 , O 2 H 2 , 

 C<jO 4 H 2 (ethane; hydrogen peroxide, oxalic acid), whilst on the other. hand, potassium 

 and sodium (perhaps also iodine at a high- temperature) contain only one atom, K, Na, 

 in the molecule in" a free state. Hence it follows that/ree affinities may exist. Granting 

 this, nothing prevents the assumption that free affinities exist in all unsaturated com- 

 pounds ; for example, two free affinities in NH 3 . If such instances of free affinities be 

 admitted, then all the possible advantages to be gained by the application of the doctrine 

 of atomicity (valency) are lost. 2.. There are instances for example, NajH where uni- 

 valent elements are combined in molecules which are more complex than Itj) and form 

 molecules, B 3 , B 4 , &c. ; this may again be either taken as evidence of the existence of 

 free affinities, or else necessitates such primary univalent elements as sodium and 

 hydrogen being considered as variable in their atomicity. 3. The periodic system of the 

 elements, with which we shall afterwards become acquainted, shows that there is a law 

 or rule for the variation of the forms of oxygen and hydrogen compounds ; chlorine is 

 univalent with respect to hydrogen, and septavalent with respect to oxygen ; sulphur is 

 bivalent to hydrogen, and sexavalent to oxygen ; phosphorus is trivalent to hydrogen 

 and pentavalent in respect to oxygen the sum is in every case equal to 8. Only carbon 

 and its analogues (for example, silicon) are quadrivalent to both hydrogen and oxygen. 

 Hence the power of the elements to change their atomicity is an essential part of their 

 nature, and therefore constant valency cannot be considered as a fundamental property. 

 4. CrystaJlo-hydrates (for instance, NaCl,2H 2 O, or NaBr,2H 2 O), double salts (such as 

 PtCl4,2KCl,H 2 SiF e , &c.), and similar complex compounds (and, according to Chap. I., 

 solutions also) demonstrate the capacity not only of the elements themselves, but also of 

 their saturated and limiting compounds, of , entering into further combination. There- 

 fore, the admission of a definite limited atomicity of the elements includes in itself an 

 admission of limitation which is not in accordance with the nature of chemical reactions. 



