AGS 4, 
NAL ORE. 
[SEPTEMBER 18, 1902 
substances of the bodies into others ; the latter is the measure of 
chemical phenomena, those of changes of bodies induced by such 
of their interactions as do involve transformations of the sub- 
stances of the interacting bodies into other substances. Since it 
is already settled for us by custom that quantities of different 
substances are to be called equal when or because they are 
equivalent gravimetrically, and as it is not to be supposed that 
we shall ever give up calling 16 kilos. of oxygen, of salt, of chalk, 
and of every other substance, however unlike, equal quantities 
of them from the gravimetric point of view, we have no choice 
but also to call molecular quantities of these substances equal 
from the chemical point of view, if the claim to coordination in 
equality of chemical with gravimetric equivalency is to be asserted 
and maintained. 
The contention that chemical equality must be regarded as of 
as clearly defined a nature as gravimetric equality becomes the 
more weighty when it is reflected that our very definite views 
concerning gravimetric equality are due solely to the law of 
conservation of mass, the evidence for and against which, I may 
remind you, is just now to be discussed by Lord Rayleigh before 
the Physical Section. The mass of one pound of sodium re- 
mains unchanged when the metal is converted into salt, washing 
soda, or borax ; if this were not the case, gravimetric equality 
would be just as definite as it is now, but physicists would have 
to argue for its general recognition in much the same way as I 
am doing now for the recognition of chemical equality. 
In further justification of this claim of chemical equality to 
coordinate rank with dynamical equality in the quantification of 
substances, it may be well to take the fact into consideration 
that the determination of the former is independent of that of 
the latter. Overlooking the difficulties of the task, let there be 
at hand or always procurable unlimited numbers of parcels of 
the different substances to be experimented upon, each of which, 
by other means than weighing, such ‘as spatial measurement, 
can be known to be equal to, or greater or less than, other 
parcels of the same substances. Suppose, now, that after many 
trials, one of a number of equal portions of sodium hydroxide 
has been found to be the quantity just necessary to interact with 
one of a number of portions of hydrochloric acid also equal 
among themselves. The products of the interaction will be 
some water and some salt. We can now have placed before us 
a parcel of sodium hydroxide equal to that previously used, 
another of hydrochloric acid also equal to that used, and the 
water and the salt obtained, and then have before us chemically 
equal quantities of four substances. Let now, by spatial 
measurement, a number of parcels of water be portioned out, 
all equal to that of the water obtained, and a number of parcels 
of salt equal to that of the salt obtained. By a series of trials 
we find a quantity of silver nitrate just sufficient to interact with 
the sodium chloride, and having, by supposition, taken this 
quantity of silver nitrate from a lot of other parcels equal to it, we 
find that one of these is just sufficient to interact with one of the 
portions of hydrochloric acid equal to that used in producing 
one of the portions of salt. Further, we find that the salt and 
the hydrochloric acid each produce a substance which is the 
same, namely, silver chloride, and in the same quantity as the 
other. Along with it in the case of the salt is sodium nitrate, 
and in the case of the hydrochloric acid, nitric acid. We can 
then find that this quantity of nitric acid is just enough to 
interact with one of those of sodium hydroxide, and thereby 
produce quantities of sodium nitrate and water, respectively 
equal to those obtained in the other interactions. If now we 
conjoin with these experiments others in which hydrogen, 
sodium and silver are each caused to combine with chlorine, 
and others in which hydrochloric acid, silver chloride and 
sodium chloride are electrolysed into these elementary sub- 
stances, evidence is obtained of such facts of chemical composi- 
tion and decomposition and of double decomposition (or what 
happens when compounds interact) as those upon which the 
science of chemistry is framed. 
In teaching chemistry the point is kept too much in the back- 
ground, if not altogether out of sight, that the chemical equality 
of quantities of different substances is independent of all other 
relations of equality between them, and that, therefore, its 
validity is not affected by the fact of its terms agreeing with 
some and not with other terms of equalities determined in other 
ways. Instead of bringing out this point the molecule of water 
is given out as being, primarily and prominently, that quantity 
which has eighteen units of mass and which measures two unit 
volumes. Both statements happen in the nature of things to be 
NO. 1716, VOL. 66] 
true, but neither of them describes the molecule. Let it be 
clearly understood from illustrative examples what is meant by 
*“chemically equal,” and there is hardly more to be said as to 
what constitutes a molecule of water than that it is the quantity 
of itchemically equal to that of some other substance presenting 
itself for comparison. ‘* Molecule” is a term of relation : it 
stands for an equal quantity, not for any particular quantity ; 
but as such it is as easy to understand and as indefinable as an 
equal volume or an equal weight of a substance. 
It is then only as colligated equalities, established by experi- 
ment, that gaseous volumes, osmotic pressures and other proper- 
ties of substances come into consideration, first as enforcing the 
truth of the conception of the indicated quantities as equal, 
and then as the means of molecular measurement without resort 
to chemical change. But of the purposes served by the colli- 
gative properties, that of giving molecular measurements with- 
out recourse to the evidence afforded by chemical change is well 
known to be of the very widest application. To determine 
chemically the molecular equalities of substances, single chemi- 
cal changes of suitable character, changes which are cases of 
double decomposition, have to be looked for; and to know 
these with the desirable degree of certainty calls for a much 
larger acquaintance with the chemical behaviour of the sub- 
stances than can usually be gained at the early stage of work 
when the knowledge of the molecule is of the utmost assistance 
in the further investigation of the nature of the substances. 
Consequently, it is nearly always through recourse to physical 
methods that the molecule is first ascertained, and then through 
the molecule the certainty acquired that some particular inter- 
action is asingle one, thus reversing the normal order of things, 
which undoubtedly is that the molecule in chemistry, however 
it may have been first determined, is recognised as such by being 
what it is in chemical change. 
I shall have been wholly misunderstood by you if you suppose 
that I would make light of the importance of the balance in 
chemical operations, or of the value of its indications in chemi- 
cal investigations. Once the weights of molecular or atomic 
quantities have been ascertained the balance becomes the most 
accurate and generally the most easily applied instrument for 
apportioning substances in these quantities. Chemical inter- 
action, to be employed in this way and without the aid of the 
balance, is practically useless, for the reason that it involves the 
destruction of the quantities it measures. Out of this depen- 
dence on the balance arises the exceeding importance of accurate 
tables of atomic weights, from which molecular weights are 
derived by addition ; but the place for these tables is not on the 
walls of the lecture-theatre, but in the laboratory pocket-book, 
and, perhaps, in the balance-room. Besides the use of the 
balance and of atomic weight tables for getting and calculating 
out molecules of different substances at pleasure, there is the 
indispensable service they perform in enabling chemical analysis 
to be carried out and applied to the solution of the problems 
offered by chemical change. The primary problem of every 
science is to find some element of sameness in the diversity of 
its phenomena, in order that they may be compared, a problem 
which was solved for chemistry to a large extent by Dalton, and 
ceased to exist when the distinction had been made between 
molecule and atom. But this having been solved, there comes 
the other problem, namely, to find definite, that is, quantitative 
differences in the midst of the uniformities, and these for the 
chemist are differences of mass or weight. Through that re- 
distribution of mass which attends chemical interactions, it has 
been possible to trace out to some extent the nature of the 
transformation of substances and develop the science on the 
lines of chemical composition and chemical constitution. Thus, 
then, the balance has become and will continue to be the neces- 
sary instrument of chemical research ; but again I would remind 
you thatit records its facts in units which are not ours, and of 
which we avail ourselves only as the means toan end. Sodium 
chloride is chemically composed, not of 3545 equal parts of 
chlorine with 2305 of the same equal parts of sodium, but of 
equal quantities of these simple substances. 
The theory of chemical molecules or equalities and their 
relations to the equalities between the weights and gaseous 
volumes of different substances were brought to light, not by 
Richter’s law of chemical combining proportions, and not by 
Avogadro’s hypothesis as to there being equal numbers of par- 
ticles in the same volume of different gases, but in the first 
place by Dalton’s atomic theory and Gay-Lussac’s law of simply 
related gaseous volumes in chemical change ; and then, much 
