43 REPORI—1874. 
CO,,CCl,. Similarly, in every case where a positive element or radical is combined 
with two negative elements or radicals, he represented the compound as composed 
of two binary compounds, thus—chloride of acetyl, 2C, H, O,, C, H, Cl,, as a com- 
pound of acetic acid and the corresponding terchloride. 
This was in perfect consistency with the mode in which ternary compounds con- 
taining one negative and two positive elements or radicals were formulated, as 
caustic potash, KO, H, O, sulphate of copper, CuO, SO,, &c.; but it lacks the prac- 
tical justification which can be given for the formula C, H,, C, O, for acetic acid ; 
for phosgene acts readily on water, forming carbonic and hydrochloric acids, an 
action which does not take place with perchloride of carbon; and it is not easy to 
see why the latter substance should be more readily attacked by water when com- 
bined with carbonic acid than when free. This difference did not escape the 
attention of Berzelius, and led him to distinguish two modes of chemical union :— 
Ist. Where the constituents were held together by the electro-chemical force, and 
wholly or partially neutralized each other, as in the oxygen and sulphur salts; and 
2nd, where a so-called ‘‘ copula” was attached by an unknown force to a substance 
without greatly modifying its chemical activity. The distinction seems arbitrary ; 
but it was not, as is usually supposed, a mere artificial bulwark to protect the 
electro-chemical theory ; it has a real and very important meaning, a meaning which 
the development of the substitution theory enables us to explain. 
The phenomena of electrolysis, upon which the Berzelian system is based, bring 
forward into great prominence one of the chemical units, viz. the equivalent; and 
the preeminent position of oxygen as the most electro-negative element made it 
most natural to select the atom of oxygen as the standard of equivalence, so that 
an equivalent of any element or radical was defined as that quantity of it which 
is equivalent to one atom of oxygen. Gay-Lussac’s law of gaseous volumes, 
which was adopted by Berzelius, and which, by a curious accident, happens to be 
true for all elements gaseous at ordinary temperatures, led to the formule H, and 
Cl, for the equivalents of hydrogen and chlorine; but although these formule 
explicitly indicate the divisibility of the equivalents of these elements, this divisi- 
bility was not recognized, and integral numbers of equivalents were alone tolerated. 
Thus hydrochloric acid was written H,Cl,, ammonia N, H,, &c., and the etymological 
meaning of the word atom was soon lost. The use of barred letters to indicate two 
atoms or one equivalent of such elements as hydrogen and chlorine further contri- 
buted to hide the important fact of their divisibility. 
The first great result of the substitution theory was to change the unit of equiva- 
lence, and to take as the standard the atom of hydrogen or of chlorine instead of 
that of oxygen; and although it would be most unjust to forget the services of 
Dumas, Gerhardt, Laurent, and Odling in this matter, the credit of removing the 
bars from H, Cl, and their comrades, and allowing the hitherto chained partners 
to walk at liberty, undoubtedly belongs mainly to our distinguished colleague and 
master Professor Williamson. 
The establishment of the water type, or (to put it in another form) the proof that 
the atom of oxygen contains two units of oxygen, inseparably united but capable of 
separate action, led the way to the explanation of all the difficulties which beset the 
theory of radicals and copulze. It at once explained how two oxides or two sulphides 
unite together*; and the idea of “ polybasic,” or, as we should now say, polyad 
atoms and radicals, was soon used to explain the existence of polybasic acids, double 
salts, acichlorides, and many other kinds of ternary compounds. 
But a fact does not cease to exist because it is explained. Quicklime and water 
unite together, although we can now explain how they do so; and a useful purpose 
may still be served by the enumeration, as in the old dualistic formule, of the pairs 
of united equivalents. Although some of these equivalents belong to the same 
atoms, it is nevertheless true that they are united in pairs. Caustic potash might 
thus be formulated, KO:, HO: or 3(K,O, H,O); phosgene gas, 3(CO,, CCl,); and 
chlorochromie acid, }(2CrC,, CrCl,). These formule are not so well suited for 
general use as those now current; but the consideration of them as accurate repre- 
sentations of facts may enable us to see that the copule of Berzelius had a real and 
* Tt does not explain the existence of double chlorides, bromides, &c. These compounds, 
apparently so similar to the double oxides and sulphides, are still unexplained. 
a 
