ON THE CONSTRUCTION OF A PHARMACOPEIA. 
569 
themselves are founded. The analysis gives the proportions of the different 
parts of a compound, and this is all we can learn in this direction with any 
certainty. What is done beyond this, and founded upon it, is the result of theory. 
We assume an atomic constitution of the body, and represent the atoms by 
symbols, but of this we have no certain knowledge, and hence the changes that 
are frequently taking place in prevailing opinions on the constitution of che¬ 
mical compounds, and the means of representing them by symbolical formulae. 
There are no material differences among chemists with regard to the composition 
of chemical compounds, analytical means being sufficiently exact to place the 
results beyond question, but great differences often arise when attempts are 
made to represent such results by atomic formulae. It must be admitted that an 
atomic formula, when adopted, affords a more clear and precise indication of 
what is intended to be conveyed than can be shortly communicated in any other 
way, but the completeness of its representation is sometimes an objection to its 
use. Thus, for instance, if we use the symbol S0 3 ,II0 for oil of vitriol, we 
either use it incorrectly, or we indicate as oil of vitriol a body that cannot be 
obtained in commerce. The sy mbol in such a case is too precise in its significa¬ 
tion, or in other words, the liquid represented is not a perfectly definite chemical 
compound, and a chemical formula is therefore inapplicable to it. There are 
many chemical substances used in medicine that are not in such a pure and de¬ 
finite state as to admit of their being correctly represented symbolically ; yet 
these, or at least some of them, may have their composition given as determined 
by analysis. The representation of composition in parts by weight is therefore 
more extensively applicable than atomic representation. Still, it cannot be de¬ 
nied that symbolical formulae, where applicable, afford great assistance in convey¬ 
ing a clear perception of the composition of bodies, and this cannot be equally 
well conveyed in any other way. If we take the analyses of two compounds, 
such as calomel and corrosive sublimate, and compare them together in the 
form in which analyses are usually expressed,—that is, in centesimal parts,—we do 
not observe that simple relationship between the two bodies which is shown by 
their symbolical formulae; nevertheless, in the one case we have the result of 
observation, and in the other of reasoning ; moreover, the reasoning is founded 
upon assumed data which are not within our means of observation. In such 
cases, however, we may render the existing relationship between the bodies more 
obvious, by giving proportional numbers corresponding with the atomic weights, 
in addition to the centesimal quantities. We may thus give the composition of 
the chlorides of mercury as follows :— 
Calomel. Corrosive Sublimate. 
Mercury, 200 84-92 . . .200 73*80 
Chlorine, 35-5 or 15-08 ... 71 0 26-20 
235-5 100-00 271 100-00 
It is obvious here that the quantity of chlorine combined with two hundred 
parts of mercury is twice as great in the case of corrosive sublimate as it is in 
that of calomel, so that this simple relationship between the two bodies is shown ; 
but the two hundred parts of mercury may represent either one atom or two 
atoms. What is gained by the adoption of this method is, that nothing is stated 
but the result of experiment, and this is reconcilable to any of the received 
theories of the constitution of the bodies. If something is lost in explicitness 
by omitting the use of the symbols, there is at least a clear gain in the unim¬ 
peachable character of what is stated. 
It must not be supposed that I am here advocating or suggesting the disuse of 
scientific names and symbolical formulae by Pharmaceutical Chemists, but only 
their disuse in the Pharmacopoeia and in physicians’ prescriptions. If the 
Pharmacopoeia describes red oxide of mercury under the name of Ilydrargyri 
