1060 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[•June £9, 1872. 
the existence of atoms ; and to show, as occasion pre¬ 
sented itself, that the weights of the molecules and the 
number of the atoms deduced by the application of this 
theory are in accordance with those which are deduced 
from chemical criteria. By this means we can measure 
the degree of confidence to he placed in the latter cri¬ 
teria ; since so-called compound equivalents do not 
suffice to determine the weight of molecules, or even 
to prove their existence, although they may he deduced 
from a single principle, the theory of the constitution of 
gases. This is the natural transition from physics to 
chemistry. 
The Professor then stated in detail how he applied the 
principles he had laid before them. He introduced his 
pupils to the study of chemistry by endeavouring to 
place them on the same level as the contemporaries of 
Lavoisier, and to teach them to appreciate the importance 
of the principle of the conservation of the weight of 
matter, showing them that this is quite independent of 
any idea of its nature or constitution. They are thus 
led to examine the ponderable composition of substances, 
so that the student passes rapidly from the epoch of La¬ 
voisier to that of Proust, and then to that of Berzelius at 
the time’when he commenced his researches on propor¬ 
tions. At this stage the same impulse is given to the 
pupil as Berzelius received on becoming acquainted with 
the hypothesis of Dalton. The latter is laid before him 
without any accessory, the use of symbols and formulae 
being introduced dogmatically. There will now arise in 
his mind the same doubts and difficulties that assailed 
Berthollet, Sir Humphry Davy and Wollaston in the 
application of Dalton’s theory, and at the same time a 
desire for an explanation of the simple relation which 
exists between the vapour volumes of bodies which react 
on one another, and of the products which are obtained. 
Now is the moment to state or recall to mind the physical 
theory of the constitution of the perfect gases. Com¬ 
mencing with a rapid glance at their general and special 
characters, he insisted that in this part of the instruction 
the mind of the student should not be diverted from the 
numbers expressing their relations by considerations of 
the variations caused by changes of temperature and 
pressure. In applying the theory of the constitution of 
gases, it will be perceived that the molecules of simple 
bodies are not always the atoms of Dalton, and a certain 
confusion will thus be produced in the mind of the be¬ 
ginner in the conception of the ideas of atoms and mole¬ 
cules. The hypothesis of Dalton can now be laid aside, 
substituting as a starting-point the theory of the relation 
of molecular weights to the vapour densities. A table 
must be prepared of the vapour density compared with 
that of hydrogen as 2 ; that is to say, the weights of 
their molecules compared with the weight of the semi¬ 
molecule of hydrogen taken as unity. We must then 
compare the composition of the molecules containing 
the same element (including or not the molecule of the 
element itself), and thence deduce the law of the exist¬ 
ence of atoms,—that is to say, the amount of each ele¬ 
ment which always enters, by whole multiples, into the 
molecules which contain them. We here have the 
atoms of Dalton, which, in the present state of the 
science, express not only all that Dalton discovered, but 
also the composition of equal volumes of their vapours, 
and in the choice of which those doubts can no longer 
arise which embarrassed Davy and Wollaston. The 
ideas of molecules and atoms suggested to the student 
by this law are devoid of all considerations of form, size, 
continuity or discontinuity; the only property indis¬ 
solubly connected with them is that of ponderability, the 
very definition of matter. 
^Recollecting that no physical theory of the constitution 
of matter had yet been advanced which thoroughly 
conformed to chemical ideas, he insisted upon the advisa¬ 
bility in teaching the molecular and atomic theory, to 
keep it free from all that is not absolutely essential, so 
that it may preserve sufficient plasticity J;o adapt itself 
to the progress of our physical and mathematical know¬ 
ledge. For this purpose he thought it useful to allow 
the student in the first place to glance at the changes in 
the hypothesis of the constitution of matter, and then to 
cause him to estimate the degree of confidence they merit 
in the actual state of our knowledge. Having thus 
placed upon a solid basis the fundamental notions of 
atoms and molecules by the comparison of the composition 
of equal volumes of the bodies in the gaseous state, it be¬ 
comes necessary to consider the difficulties which arise in 
the application of these notions when the vapour densities- 
are wanting ; he explained and justified the use of the 
various auxiliary criteria to which we have resource in 
these cases, proving them in the first instance by the 
touchstone of the theory of Avogadro and Clausius, by 
showing that they gave results in accordance with that 
theory whenever the two methods can be employed 
simultaneously. 
He believed that we should never lose sight of the 
starting point, nor give the formulae of all compounds as 
of equal probability. “ It is not by concealing the ob¬ 
scurity of these questions that we shall enlighten the 
student; on the contrary, we should estimate each fact at 
its true value by showing him that our science does not 
merit an equal degree of confidence on all points.” This 
forms the introduction, the preparation for the study of 
the transformations which matter undergoes; the real 
object and aim of our science. 
The comparison of the atomic composition of molecules 
has led chemists to the law of substitution, to the theory 
of types of Dumas, then to that of Williamson and Ger- 
hardt, and lastly to the theory of the different valency of 
atoms and their modes of union, or the so-called theory 
of atomicity which includes the former. Although at 
present it is impossible, in teaching chemistry, entirely 
to eliminate this latter theory, which gives a summary 
of several laws, and guides us ordinarily in the co-ordi¬ 
nation and even prevision of a large number of facts, yet 
it is difficult to keep it within just bounds so as to avoid 
infusing into the mind of the beginner illusions which 
are dangerous for his intellectual education. In order to 
avoid this, it is advisable to bear in mind the progress of' 
this doctrine and the actual phase of development which 
it has at present reached. It is still far from being a 
complete and well-established theory, but is in a state of 
transition; for although doubtless it embraces a large 
number of facts, as yet it does not embrace them all. It 
is only a partial representation of the reality, and that 
from a restricted point of view, showing but little rela¬ 
tion to our views of the constitution of matter, for it is 
the result of a comparison of diverse facts expressed by 
means of the atomic and molecular theory. It is con¬ 
venient, therefore, to consider each part of this doctrine 
exclusively in relation to the group of facts which has 
suggested it. 
It is unadvisable to define the valency of atoms as a 
property inherent in them, and then to deduce as a co¬ 
rollary their different modes of union ; on the contrary, 
it is preferable to regard each portion of this doctrine as 
a deduction from the observation and comparison of a 
determinate group of facts, until an opportunity offers to 
unite these fragments into one whole, not forgetting, 
however, to notice the gaps which exists, never going 
beyond what the facts themselves suggest, and never ap¬ 
plying to all bodies indiscriminately, the laws which suit 
only a single group. For instance, we must not pass 
over in silence the facts that whilst certain elements are 
bi- tetra- or even hexa-valent, others are tri- and penta- 
valent; but the pupil should be prevented from acquir¬ 
ing mechanical and geometrical ideas of the cause and 
effects of the valency of atoms, by frequently reminding 
him that chemical facts show nothing about the size, 
form, continuity, or relative position of atoms. If we 
are sometimes obliged to employ the expression, “rela¬ 
tive position of atoms in the molecules,” and even to re¬ 
present them graphically, we must warn the student that 
