SODIUM CHLORIDE-BERTHOLLET'S LAWS 407 



the acidity or basicity), at all events, the composition l of their com 

 pounds. This forms the substance of the conception of the valency or 



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.,, CH 4 , 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. And carbon, nevertheless, is an example of an element which preserves its 

 atomicity to a greater degree than most of the other elements. Nitrogen in NH 5 , 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 oxgen = 2, nitrogen has an uneven atomicity (1, 3, 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 , KNS, &c.), then it could not be foreseen from this that it would form SO 2 , 

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

 acknowledged as greater than 2. Thus in SOo, sulphurous anhydride, there are a multi- 

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

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

 be established as the basis for the study of the elements, although it gives an easy possi- 

 bility for grasping many analogies. Although a definite atomicity cannot be considered 

 us a radical property of atoms and elements, it may, however, be made use of with great 

 advantage in investigating the compounds of such elements as carbon, because carbon in 

 all its usual compounds, and especially in the saturated compounds and those approach- 

 ing them, acts as a quadrivalent element. I consider the four following as the chief 

 obstacles to acknowledging the atomicity of the elements as a primary conception for the 

 consideration of the properties of the elements : 1. Such univalent elements as H, Cl, 

 tVc., appear in a free state as molecules H x >, CL, &c., and are consequently like the uni- 

 valent radicles CH 5 , OH, CO 2 H, &c., which, as might be expected, appear as C 2 H 6 , 

 O 2 H 2 , C 2 O 4 H.> (ethane, hydrogen peroxide, oxalic acid), whilst on the other hand, potas- 

 sium 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 free affinities may exist. 

 And then nothing obliges us not to admit free affinities in all unsaturated compounds ; for 

 example, it may be said that in CoH 4 each atom of carbon is bound to the other by one 

 affinity, two other affinities holding the hydrogen, and that the fourth affinity of each 

 carbon atom is free. 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, Na 2 H where univalent elements are 

 gathered together in molecules which are more complex than R 2 , and form molecules, 

 R 3 , R 4 , &c. ; which may again be either taken as evidence of the existence of free affini- 

 ties, 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 penta- 

 valent 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 

 property of the elements to change their atomicity lies in the essence of their nature, and 

 consequently atomicity cannot be considered as a fundamental property. 4. Crystallo- 

 hydrates (for instance, NaCl,2H 2 O, or NaBr,2H 2 O), double salts (for instance, PtCl 4 ,2KCl, 

 H 2 SiF 6 , &c.), and similar complex compounds (and, according to Chap. L, solutions also) 

 demonstrate the capacity not only of the elements themselves, but also of their saturated 

 and limiting compounds, of entering into further combination. Therefore the admission 

 of a definite limited atomicity of the elements includes in itself an admision of limita- 

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



