I assume that most of those whom I address are teachers, and that you
have been drawn here by a desire to be instructed in the best methods of
teaching physical science. It has therefore seemed to me that I might
render a real service, in this introductory address, by giving the
results of my own experience and reflection on this subject; and my
thoughts have been recently especially directed to this topic by the
discussion in regard to the requisites for admission, which during the
past year have actively engaged the attention of the faculty of this
college.
[P] An address delivered at the opening of the Summer School of
Chemistry at Harvard College, July 7, 1884.
At the very outset of this discussion we must be careful to make a clear
distinction between instruction and education--between the acquisition
of knowledge and the cultivation of the faculties of the mind. Our
knowledge should be as broad as possible, but, in the short space of
human life, it is not, as a rule, practicable to cultivate, for
effective usefulness, the intellectual powers in more than one
direction.
Let me illustrate what I mean from that department of knowledge which is
at once the most fundamental and the most essential. I refer to the
study of language. No person can be regarded as thoroughly educated who
has not the power of speaking and writing his mother-tongue accurately,
elegantly, and forcibly; and scholars of the present day must also
command, to a considerable extent, both the French and the German
languages. These three languages, at least, are the necessary tools of
the American scholar, whatever may be the special field of his
scholarship, and his end is gained if he has acquired thorough command
of these tools. But if he goes further, and studies the philology of
these languages, their structure, their derivation, their literature,
the study may occupy a lifetime, and be made the basis of severe
intellectual training. More frequently, and as most scholars think more
effectually, such linguistic training is obtained by the study of the
ancient languages, especially the Latin and the Greek, and no one
questions the value and efficiency of this form of mental discipline.
But obviously such a preparation is not necessary for the use of the
modern languages as tools, or in order to acquire a knowledge of ancient
history, of the modes of ancient life, or the results of ancient
thought. In recent discussions a great deal has been said about the
value of classical learning, and it has been argued that no man could be
regarded as thoroughly educated who had never heard of Homer or Virgil,
of Marathon or Cannae, of the Acropolis of Athens or the Forum of Rome.
Certainly not. But all this knowledge can be acquired without spending
six years in learning to read the Latin and Greek authors in the
original, or in writing Latin hexameters or Greek iambics. The
discipline acquired by this long study is undoubtedly of the highest
value, but its value depends upon the intellectual training which is the
essential result, and not upon the knowledge of ancient life and
thought, which is merely an incident.
Now, this same distinction, which I have endeavored to illustrate on
familiar ground, must not be forgotten in considering the relations of
physical science to education. Physical science may also be studied from
two wholly different points of view: First, to acquire a knowledge of
facts and principles, which are among the most important factors of
modern life; secondly, as a means of developing and training some of
the most important intellectual faculties of the mind--for example, the
powers of observation, of conception, and of inductive reasoning.
The experimental sciences must often be studied chiefly from the first
point of view. If no man can be regarded as thoroughly educated who is
ignorant of the outlines of Roman and Greek history: one who knows
nothing of the principles of the steam-engine, or of the electric
telegraph, is certainly equally deficient. I do not question that in our
high-schools the physical sciences must be taught, for the most part, as
funds of useful knowledge, and in regard to such teaching I have only a
few remarks to make. Assuming that information is the end to be
attained, the best method of securing the desired result is to present
the facts in such a way as will interest the scholar, and thus secure
the retention of these facts by his memory. I think it a very serious
mistake to attempt to teach such subjects by _memoriter_ recitations
from a text-book, however well prepared. This method at once makes the
subject a task; and, if in addition the preparation for an examination
is the great end in view, it is wonderful how small is the residuum
after the work is done. Such subjects can always be made intensely
interesting if presented by lectures, with the requisite illustrations,
and I do not believe that the cramming process required to pass an
examination adds much to the knowledge previously gained. Many teachers,
finding that the parrot-like learning of a text-book is unprofitable,
attempt to make the exercise more valuable by means of problems--usually
simple arithmetical problems--depending upon principles of physics or
chemistry. And there can be no doubt that such problems do serve to
enforce the principles they illustrate; but I am afraid they also more
frequently, by disgusting the student, stand in the way of the
acquisition of the desired knowledge.
It must not be forgotten, in studying the results of science, that the
facts are never fully learned unless the learner is made to understand
the evidence on which the facts rest. The child who reads in his
physical geography that the world revolves on its axis, learns what to
him is a mere form of words, until he connects this astronomical fact
with his own observation that the sun rises in the east and sets in the
west; and so the scholar who reads that water is composed of oxygen and
hydrogen has acquired no real knowledge until he has seen the evidence
on which this fundamental conclusion rests. Let, then, the sciences be
taught as they have been in schools, as important parts of useful
knowledge, but let them so be taught as to engage the interest of the
scholar, and to direct his attention to the phenomena of Nature.
All this, however, is not scientific culture, in the sense in which I
have constantly used the term, and does not afford any special training
for the intellectual faculties. For myself, I do not desire any study of
natural history, chemistry, or physics from this point of view as a
preparation for college; simply because, with the large apparatus of the
university, all these subjects can be presented more effectively, and be
made more interesting, than is possible in the schools. What I desire to
see accomplished by our schools is a training in physical science,
comparable in extent and efficiency with that which they now accomplish
in the ancient languages. And this brings me to another topic, namely,
scientific culture as a system of mental training.
Before attempting to state in what scientific culture consists, we shall
do well, even at the expense of some repetition, to show that what often
passes for scientific culture is far different from the system of
education which we have so constantly advocated. The acquisition of
scientific knowledge, however extensive, does not in itself constitute
scientific culture, nor is the power of reproducing such knowledge, at a
competitive examination, any test of real scientific power.
Nevertheless, the examination papers which have been published by the
universities of England and of this country show that this is the sole
test of scientific scholarship on which most of these universities rely,
in awarding their highest honors to students in physical science. The
power of so mastering a subject as to be able to reproduce any portion
of it with accuracy, completeness, and elegance, at a written
examination, is the normal result of literary, not of scientific,
culture, and the power is of the same order, whether the subject-matter
be philology, literature, art, or science. Indeed, scientific are, as a
rule, much less adapted than literary subjects to the cultivation of
this power. Moreover, it is also true that scholars, having attained to
a very high degree of scholarship, may not possess this power of stating
clearly and concisely the knowledge they actually possess. We have all
of us known eminent men, possessing in a very high degree the power of
investigating Nature, who have been wholly unable to state clearly the
knowledge they have themselves discovered. Great harm has been done to
the cause of scientific culture by attempting to adapt the well-tried
methods of literary scholarship to scientific subjects: for, as I have
said in another place, competitive examinations are no test of real
attainment in physical science.
Let me not be understood as disparaging the retentive memory and power
of concentration which enable the student to reproduce acquired
information with accuracy, rapidity, and elegance. This is a power of
the very highest order, and is the result of the cultivation to a high
degree of many of the noblest faculties of the mill. And I wish to
enforce is, that success in such examinations is no indication of
scientific culture, properly so called.
What, then, are the tests of true scientific scholarship? The answer can
be made perfectly plain and intelligible. The real test is the power to
study and interpret natural phenomena. As in classical scholarship the
true test of attainment is the power to interpret the delicate shades of
meaning expressed by the classical authors, so in science the true test
is the power to read and interpret Nature; and this last power, like the
other, can as a rule only be acquired by careful and systematic
training. As some men have a remarkable facility for acquiring
languages, so also there are men who seem to be born investigators of
Nature; but by most men such powers can only be acquired through a
careful training and exercise of the faculties of the mind, on which
success depends. No man would be regarded as a classical scholar,
however broad and extended his knowledge, if that knowledge had been
acquired solely by reading English translations of the classical
authors, however excellent. So, no man can be regarded as a scientific
scholar whose knowledge of Nature has been solely derived from books.
In either case the real scholar must have been to the fountain-head and
drawn his knowledge from the original sources. In order, then, to
discover how scientific culture must be gained, we must consider the
conditions on which the successful study and interpretation of Nature
depend.
Of the powers of the mind called into exercise in the investigation of
Nature, the most obvious and fundamental is the power of observation. By
power of observation is not meant simply the ability to see, to hear, to
taste, or to smell with delicacy, but the power of so concentrating the
attention on what we observe as to form a definite and lasting
impression on the mind. There are undoubtedly great differences among
men in the acuteness of their sensations, but successful observation
depends far less upon the acuteness of the senses than on the faculty of
the mind which clearly distinguishes and remembers what is seen and
heard. We say of a man that he walks through the world with his eyes
shut, meaning that, although the objects around him produce their normal
impression on the retina of his eye, he pays no attention to what he
sees. The power of the naturalist to distinguish slight differences of
form or feature in natural objects is simply the result of a habit,
acquired through long experience, of paying attention to what he sees,
and the want of this power in students who have been trained solely by
literary studies is most marked.
An assistant, who was at the time conducting a class in mineralogy, once
said to me: "What am I to do? One of my class can not see the difference
between this piece of blende and this piece of quartz" (showing me two
specimens which bore a certain superficial resemblance in color and
general aspect). My answer was, "Let him look until he can see the
difference." And, after a while, he did see the difference. The
difficulty was not lack of vision, but want of attention.
The power of observation, then, is simply the power of fixing the
attention upon our sensations, and this power of fixing the attention is
the one essential condition of scholarship in all departments of
learning. It is a power which ought to be cultivated at an early age,
and in a system of scientific culture the sciences of mineralogy and
botany afford the best field for its culture, and I should therefore
place them among the earliest studies of a scientific course. Minerals
and plants may be profitably studied in the youngest classes of our
secondary schools, but they should be studied solely from specimens,
which the scholar should examine until he can distinguish all the
characteristics of form, feature, or structure. I am told that in many
of our secondary schools both mineralogy and botany are studied with
great success and interest in the manner I have indicated. But a mistake
is frequently made in attempting to do too much. With mineralogy or
botany as classificatory sciences, our secondary schools should have
nothing to do. The discrimination between many, even of the commonest,
species of minerals or plants depends upon delicate distinctions which
are quite beyond the grasp of young minds, and the study of botany
frequently loses all its value, through the ambition of the teacher to
embrace so much of systematic botany as will enable scholars "to analyze
plants."
If a child, twelve or fourteen years of age, is made to observe the
characteristic qualities of a few common minerals so as to enable it to
recognize them in the rocks, and is likewise led to examine the
structure of a few familiar flowers, not only will a new power have been
acquired, but a new interest will have been added to life.
Of course, the faculty of observation thus early exercised in childhood
only attains the highest degree of development after long experience and
continued practice. The acuteness which practice gives is frequently
very remarkable, and rude men often surprise us by the extent to which
their power of observation has been cultivated in certain special
directions. The sailor who recognizes the outlines of to him a
well-known coast, where the ordinary traveler sees nothing but a bank of
clouds, or the miner who recognizes in the rock indications of valuable
ores, are illustrations which may give a clearer conception of the
nature of the power we have been attempting to describe.
Naturally following the power of observation in the order of education
is the power of conception with the cognate power of abstraction; that
is, the power of forming in the mind distinct and accurate images of
objects, and relations, which have been previously apprehended either by
direct observation, or through description; and also the power of
confining the attention to certain features which these images may
present to the exclusion of all others. This is a power which depends
very greatly on the imagination and is capable of being cultivated to a
very high degree. There is no study which is so well suited to the
training both of the powers of conception and of abstraction as the
study of geometry.
To this end the study of geometry should be begun at an early period in
school-life, and it should be studied at first not as a series of
propositions logically connected, but as a description of the properties
and relations of lines, surfaces, and solids--what has sometimes been
called "the science of form." A text-book prepared on this idea by Mr.
G. A. Hill forms an admirable introduction to the study.
I esteem very highly the system of geometry of Euclid, either in its
original form or as it has been modified by modern writers, as a means
of developing the logical faculty. The completeness of the proof of the
successive propositions and their mutual dependence by means of which,
as on a series of steps, we mount from simple axiomatic truths to the
most complex relations, furnish an admirable discipline for the
reasoning power; but too often the whole value of this discipline is
lost by the failure of the pupil to form a clear conception of the very
relations about which he is reasoning, and the study becomes an exercise
of the memory and nothing more. Often have I seen a conscientious and
faithful student draw an excellent figure, and write out an accurate
demonstration, without noticing that the two were not mated; and in a
recent meeting of teachers of our best secondary schools it was gravely
asserted that solid geometry is the most difficult study with which the
teachers had to deal. In solid geometry, however, the reasoning is no
more difficult than in plane geometry, but the conceptions are far more
complex, and, if the teacher insisted that the pupil should not take a
single step until his conceptions were perfectly clear, all the
difficulties would disappear. Of this I am fully persuaded, for I have
had to encounter the same difficulties over and over again in teaching
crystallography. In beginning the study of geometry, of course the power
of conception should be helped in every possible way. Let your pupil
find out by actual measurement that the sum of the angles of a triangle
is equal to two right angles, and he will easily discover the proof of
the proposition himself. So, also, if he actually divides with his knife
a triangular prism made from a potato or an apple into three triangular
pyramids, he will find no difficulty in following the reasoning on which
the measurement of the solid contents of a sphere depends. Let me assure
teachers that the study of geometry, taught as I have indicated, is a
most valuable introduction to the study of science. But, as it has been
usually taught as a preparation for college, it is almost worthless in
this respect, however valuable it may be as a logical training.
I consider practice in free-hand drawing from natural objects a most
valuable means of training both the power of observation and the power
of conception, besides giving a skill in delineation which is of the
greatest importance to the scientific student. Accuracy of drawing
requires accuracy in observation, and also the ability to seize upon
those features of the object which are the most prominent and
characteristic. Hence, in a course of scientific training, the
importance of practice in drawing can hardly be exaggerated, and it
should be made one of the most important objects of school-work from an
early period.
To the scientific student the powers of observation and conception are
not sought as ends in themselves, but as means of studying Nature. The
precise portions of this wide field to which the attention of the
student shall be directed will be determined by many circumstances, and
it is not our purpose in this address to lay down a plan of study. To
most students the natural history subjects offer the most attractive
field; but all, I think, will admit that the experimental sciences
should form a considerable portion, at least, of the course of all
scientific students, whatever specialty may subsequently be chosen. That
on which I desire particularly to dwell is the spirit in which all these
studies should be pursued; and I can best illustrate what I mean by
confining my remarks to that subject in which I am most interested, and
in regard to which I have the greatest experience.
In a course of scientific study, chemistry can not be dissociated from
physics, and the two sciences ought to be studied to a great extent in
connection with each other. Not only does the philosophy of chemistry
rest upon physical conceptions; but, moreover, chemical methods involve
physical principles. There is, however, a distinction to be made; for,
while some of the departments of physics are best studied as a
preparation for chemistry, there are other subjects which are best
deferred until the student has some knowledge of chemical facts. Among
the preliminary subjects we should mention elementary mechanics,
including hydrostatics and pneumatics, and also thermotics; while
electricity, acoustics, and optics, including the large subject of
radiant energy, may well be deferred until after the study of chemistry.
In the study both of chemistry and physics there are of course two
definite objects to be kept in view: In the first place, a knowledge of
the facts of the science is to be acquired; in the second place, the
student must learn by experience how these facts have been discovered.
It would be obvious, from a moment's reflection, that a knowledge of the
circumstances under which the facts of Nature are revealed to the
student is essential to a complete apprehension of the facts themselves.
The child who is taught that the earth moves in an elliptical orbit
around the sun in one year does not in the least grasp the wonderful
fact thus stated, and will not come to realize it until he connects the
statement with the nightly procession of the stars in the heavens. And
it is just such a connection as this which the teacher must seek to
establish in all scientific teaching. In experimental science such a
connection is most readily established in the mind of the student by
means of a series of well-arranged experiments, which each one repeats
for himself at the laboratory table. Obviously, however, it is
impossible, in a limited course of teaching, to go over the whole ground
of chemistry and physics in this way, or even over that small portion of
the ground with which the average scientific student can expect to
become acquainted. Nor is this necessary; for, after one has realized
the connection between phenomena and conclusion in a number of
instances, the mind will fully comprehend that a similar connection
exists in other cases, and will understand the limitations with which
scientific conclusions are to be received.
Hence, it seems to me that, in teaching chemistry or physics, it is best
to combine a course of lectures which should give a broad view of the
whole ground with a course of laboratory instruction, which must
necessarily be more or less restricted. Experimental lectures are, I am
convinced, much the best way of presenting these subjects as systematic
portions of knowledge. It is not necessary that the lectures should be
formal, but it is all-important that they should be given in such a way
that the interest of the student should be awakened, and that they
should be fully illustrated by specimens and experiments. What we read
in a book does not make one half the impression that is produced by the
words of a living teacher, nor can we realize the facts unless we see
the phenomena described. There is undoubtedly an advantage to be gained
in subsequently reviewing the subject as presented in a good text-book,
and such a book may be of great use in preparation for an examination.
But how far examinations are of value in enforcing the acquisition of
knowledge of an experimental science is a question on which I feel a
grave doubt. Certainly their value is very small if, as is too
frequently the case, they lead the student to defer all effort to make
his own the knowledge presented in the lectures, until a final cram.
The management of lectures, text-books, and examinations, will not,
however, offer nearly so great difficulties to the teacher as the
management of the parallel experimental course of laboratory teaching.
In the last the methods are less well tried and demand of the teacher a
very considerable amount of invention and experimental skill. To follow
mechanically any text-book would result in a loss of the proper spirit
with which the course should be conducted and which constitutes its
chief value. No experiments are so good as those which have been devised
by the teacher, or, still better, by the pupils themselves. A mere
repetition of a process, according to a definite description, has no
more value than a repetition of a form of words in an ordinary school
recitation. The teacher must make sure that the student fully
understands what he is about, and comprehends all the connections
between observations and conclusions which it is his aim to establish.
Moreover, he must constantly encourage his students to think and work
for themselves, and direct them in the methods of inductive reasoning.
The failure of an experiment may be made most instructive if the student
is led to discover the cause of the failure. A leak in his apparatus may
be turned to a similar profit if the student is shown how to discover
the leak, by carefully eliminating one part after another until the weak
point is made evident.
The direction of an experimental laboratory is no easy task. The teacher
must make each man's work his own, and follow his processes of thought
as well as his experiments with the most careful attention. With large
classes much time can be saved by going through each process on the
lecture-room table and giving the directions to the class as a whole;
but this does not supersede the personal attention and instruction which
each student requires at the laboratory table. Moreover, in laboratory
teaching the teacher must rely, as we have said, on his own resources,
and but few aids can be given. There are books, however, which will help
the teacher to prepare himself for his work, and I am happy to say that
a book entitled "The New Physics," prepared by my colleague, Professor
Trowbridge, is now being printed, which I hope will greatly promote the
laboratory teaching of physics. Nichols's abridgment of Eliot and
Storer's Manual has long served a similar valuable purpose in chemistry,
and there are many excellent works on "Qualitative Analysis," a study
which is admirably adapted to develop the power of inductive reasoning.
There is, however, a danger with all laboratory manuals, which must be
sedulously avoided, and the danger is generally greater the more precise
the descriptions. They are apt to induce mechanical habits which are
fatal to the true spirit of laboratory teaching. Not long ago I asked a
student, who was working in our elementary laboratory, what he was
doing. He answered that he was doing No. 24, and immediately went to
find his book to see what No. 24 was. I fear that a great deal of
laboratory work is done in a way which this anecdote illustrates, and,
if so, it is a mere waste of time.
When teaching qualitative analysis it was always with me a constant
struggle to prevent just such a result, and many of the excellent tables
which have been prepared to facilitate analysis simply encourage the
evil practice. It is an error to which college students, with their
exclusively literary preparation, are especially liable, and I have no
question that the proper conduct of our laboratories would be made much
easier if the students came with a previous scientific training.
Thus far I have dealt solely with generalities, and my object has been
not so much to give definite directions as to make suggestions which
might lead to better systems of teaching. The details of these systems
may vary widely, and yet all may lead to the desired result if only the
true spirit of scientific teaching is preserved, and a teacher's own
system is generally the best system for him. This leads me to explain my
own system of teaching chemistry--which presents some novelties that may
be of interest, and, although it has been worked out in detail in the
revised edition of the "New Chemistry," just published, still a few
words of explanation may be of value at this time in setting forth its
salient points.
Chemistry has been usually defined as the science which treats of the
composition of bodies, and in most text-books the aim has been to
develop the scheme of the chemical elements, and to show that, by
combining these elements, all natural and artificial substances may be
prepared. In the larger text-books, which aim to cover the whole ground
and to describe all known substances, such a method is both natural and
necessary. But, as an educational system, this mode of presenting the
subject is, as a rule, profitless and uninteresting. The student becomes
lost amid details which he can only very imperfectly grasp, and the
great principles of the science, as well as their relations to cognate
departments of knowledge, are lost sight of. Moreover, the system is
unphilosophical, because it presents the conclusions of chemistry before
the observations on which they are based. Any one who has attempted to
teach chemistry from the ordinary elementary text-books must have
experienced the truth of what I have said.
A student learns a lesson about sodium and the various salts of this
metal, and, after glibly reciting the words of the text-book, how much
more does he know of the real relations of these bodies than he did
before? Thus: "Chloride of sodium, symbol NaCl. Crystallizes in cubes.
Soluble in water. Solubility only slightly increased by heat. Generally
obtained by evaporation of sea-water in pans. Also found in beds in
certain geological basins, from which it is extracted by mining. When
acted upon by sulphuric acid, hydrochloric acid is evolved and sodic
sulphate is formed, according to the following reaction," and so on. I
have known a student to recite all this and a great deal more, without
ever dreaming that he had been eating chloride of sodium on his food,
three times a day at least, since he was born.
Now, the rational system of teaching chemistry is first to present to
the scholar's mind the phenomena of Nature with which the science deals.
Lead him to observe these phenomena for himself; then show him how the
conclusions which together constitute that system of knowledge we call
chemistry have been deduced from these fundamental facts. My plan is to
develop this system in the lecture-room in as much detail as the time
allotted will permit; to illustrate all the points by experiment, and in
addition to explain more in detail carefully selected fundamental
experiments, which the student subsequently repeats in the laboratory
himself. Thus I make the lecture-room instruction and the laboratory
demonstration go hand in hand as complementary parts of a single course
of teaching.
I begin by directing the student to observe for himself the properties
of bodies by which substances are distinguished. I place in his hands a
bit of roll-brimstone. He first notices the color, the hardness, the
brittleness, and the electrical excitability of this material. He next
determines its density, its melting-point, its point of ignition, and,
if practicable, its boiling-point. Then he treats the brimstone with
various solvents, and finds that, while insoluble in water or alcohol,
it dissolves readily in sulphide of carbon. Afterward he evaporates the
solution thus made, and obtains definite crystals, whose forms he
studies, and compares with the forms of the crystals of the same
material which he also makes by fusion. Lastly, he observes the
remarkable change which follows when fused brimstone is heated above its
melting-point, and also the peculiar plastic condition which the
material assumes when the thickened mass is poured into water. He will
thus be led to see that the same material may assume different states,
and gain a clear conception of the substance we call sulphur. After this
I give the student pieces of two metals which externally resemble each
other, like lead and tin, in order that, after making another series of
observations and experiments, he may come to understand on what
comparatively slight differences of properties the distinction between
substances is frequently based. A comparison is next made of the
properties of two closely-allied liquids, like methylic and ethylic
alcohol; and by this time the student attains sufficient skill in
experimenting to make a comparison between two aeriform substances, like
oxygen gas and carbonic dioxide.
After more or less of such preliminary work, we are prepared to take up
the subject-matter of chemistry. In the broad fields of Nature what
portion does this science cover? Natural phenomena may obviously be
divided into two great classes: First, those changes which do not
involve a transformation of substance; and, secondly, those changes
whose very essence consists in the change of one or more substances into
other substances having distinctive properties. The science of physics
deals with the phenomena of the first class; the science of chemistry
with those of the last. Any phenomenon of Nature which involves a change
of substance is a chemical change, and in every chemical change one or
more substances, called the factors, are converted into another
substance or into other substances called the products. The first point
to be made in teaching chemistry is, that a student should realize this
statement, and a number of experiments should be shown in the
lecture-room and repeated in the laboratory illustrating what is meant
by a chemical change.
Here, of course, arises a difficulty in finding examples which shall be
at once simple and conclusive, for in almost all natural phenomena there
is a certain indefiniteness which obscures the simple process. The
familiar phenomena of combustion are most striking examples of this
fact, and men were not able to penetrate the mist which obscured them
until within a hundred years. To find chemical processes whose whole
course is obvious to an unpracticed observer, we are obliged to resort
to unfamiliar phenomena.
A very simple example of a chemical process is a mixture of sulphur and
zinc in atomic proportions, which, when lighted with a match, is rapidly
converted into white sulphide of zinc, with appearance of flame. Another
example, a mixture of sulphur and fine iron-filings, which, when
moistened with a little water, rapidly changes into a black sulphide of
iron. Then some copper-filings, which, when heated on a saucer in the
open air, slowly change into black oxide of copper. Then a bit of
phosphorus, burned in dry air under a glass bell, yielding a white
oxide. Next, some zinc, dissolved in diluted sulphuric acid, yielding
hydrogen gas and sulphate of zinc. Then, a solution of chloride of
barium added to a solution of sulphate of soda, giving a precipitate of
sulphate of baryta, and leaving in solution common salt, which can be
recovered by evaporating the filtrate.
In all these examples the student should be made to see and handle all
the factors and all the products of each process, and the experiments
should be selected so that he may become familiar with the different
conditions under which substances appear, and with various kinds of
chemical processes. He should also be made clearly to distinguish
between the essential features of the process and the different
accessories, which may be more or less accidental--such, for example, as
the water used in determining the combination of iron and sulphur, or
the flame which accompanies combustion.
After a clear conception has been gained of a chemical process, with its
definite factors and definite products, we are prepared for the next
important step. Every chemical process obeys three fundamental laws:
The Law of Conservation of Mass.
The Law of Definite Proportions.
The Law of Definite Volumes.
According to the first law, the sum of the weights of the products of a
chemical process is always equal to the sum of the weights of the
factors. This law must now be illustrated by experiments, and
approximate quantitative determinations should be introduced thus early
into the course of study. All that is required for this purpose is a
common pair of scales, capable of weighing two or three hundred grammes,
and turning with a decigramme. We use in our laboratory some
platform-scales, made by the Fairbanks Company, which are inexpensive,
and serve a very useful purpose.
A very satisfactory illustration of the law of conservation of mass can
be obtained by inserting in a glass flask a mixture of copper-filings
and sulphur in atomic proportions. The glass flask is first balanced in
the scale-pan; then removed and gently heated until the ignition which
spreads through the mass shows that chemical combination has taken
place. The flask is lastly allowed to cool, and on reweighing is found
not to have altered in weight.
For a second experiment, a bit of phosphorus may, with the aid of some
simple contrivance, be burned inside a tightly-corked glass flask, of
sufficient volume to afford the requisite supply of oxygen. Of course,
on reweighing the flask, after the chemical change has taken place, and
the bottom of the flask covered with the white oxide formed, there will
be no change of weight, and this experiment may be made to enforce the
truth that, in this example of combustion at least, the chemical process
is attended with no loss of material. Open now the flask, and air will
rush in to supply the partial vacuum, proving that in the process of
combustion a portion of the material of the air has united to form the
white product.
Make now a third experiment as an application of the general principle
which has been illustrated by the previous experiments. Burn some finely
divided iron (iron reduced by hydrogen) on a scale-pan, and show that
the process is attended by an increase of weight. What does this mean?
Why, that some material has united with the iron to form the new
product. Whence has this material come? Obviously from the air, for it
could come from nowhere else. And thus, besides illustrating the first
of the above laws, this experiment may be made to furnish an instructive
lesson in regard to the relations of the oxygen of the atmosphere to
chemical processes.
The second law declares that in every chemical process the weights of
the several factors and products bear each to the others a definite
proportion. This law must next be made familiar by experimental
illustrations. A weighed amount of oxide of silver is placed in a glass
tube connected with a pneumatic trough. The tube is gently heated until
the oxide is decomposed and the oxygen gas collected in a glass bottle
of sufficient size. The metallic silver remaining in the tube is now
reweighed, and the volume of the oxygen gas in the bottle measured, and
from the volume of the gas its weight is deduced. The measurement is
easily made by simply marking with a gummed label the level at which the
water stands in the bottle. If, now, the bottle is removed from the
pneumatic trough and the weight of water found which fills the bottle to
the same height, the weight of the water in grammes will give the volume
of the gas in cubic centimetres, and, knowing the weight of a cubic
centimetre of oxygen, we easily calculate the weight of this gas
resulting from the chemical process. We have now the weights of the
oxide of silver, the silver, and the oxygen, the one factor and the two
products of the chemical process, and, by comparing the results of
different students making the same experiment, the constancy of the
proportion will be made evident to the class.
For a second illustration of the same law, the solution of zinc in
dilute sulphuric acid, yielding sulphate of zinc and hydrogen gas, may
be selected, and the weight of the hydrogen, estimated as in the
previous example, shown to sustain a definite relation to the weight of
the zinc dissolved.
Again, silver may be dissolved in nitric acid, and the weight of the
nitrate of silver obtained shown to sustain a definite relation to the
weight of the metal.
Or, still further, as an experiment of a wholly different class, a known
weight of chloride of barium may be dissolved in water, and, after
precipitation with sulphuric acid, the baric sulphate collected by
filtration and weighed, when the definite relation between the weight of
the precipitate and the weight of the chloride of barium will appear.
For a last experiment let the student neutralize a weighed amount of
dilute hydrochloric acid with aqua ammonia, noting approximately the
amount of ammonia required. Let him now evaporate the solution on a
water-bath, and weigh the resulting saline product; taking next the same
quantity of hydrochloric acid as before, and, having added twice the
previous quantity of ammonia, let him obtain and weigh the resulting
salammoniac as before. A third time let him begin with half the
quantity of hydrochloric acid, and, adding as much ammonia as in the
first case, again repeat the process. It is obvious what the result of
these experiments must be; but without telling the student what he is to
expect, it will be a good exercise to ask him to draw his own inferences
from the results. Of course, he must previously have so far been made
acquainted with the properties of hydrochloric acid and ammonia as to
know that the excess of either would escape when the saline solution is
evaporated over a water-bath. But with this limited knowledge he will be
able to deduce the law of definite proportions from the experimental
results thus simply obtained.
The third of the fundamental laws of chemistry stated above (generally
known as the law of Gay-Lussac) declares that, when two or more of the
factors or products of a chemical process are aeriform, the volumes of
these gaseous substances bear to each other a very simple ratio. Here,
again, numerous experiments may be contrived to illustrate the law.
Water, when decomposed by electricity, yields hydrogen and oxygen gases
whose volumes bear to each other the ratio of two to one. When
hydrochloric-acid gas is decomposed by sodium amalgam, the volume of the
original gas bears to that of the residual hydrogen the ratio also of
two to one. When ammonia is decomposed by chlorine, the volume of the
resulting nitrogen gas is one third of that of the chlorine gas
employed.
Having illustrated these three general laws, attention should be
directed to the fact that the nature of a chemical process and the laws
which it obeys are results of observation and involve no theory
whatsoever. On these facts the science of chemistry is built. The modern
system of chemistry, however, assumes what is known as the molecular
theory, and by means of this theory attempts to explain all these facts
and show their mutual relations. Here the distinction between fact and
theory must be insisted upon, and also the value of theory for
classifying facts and directing observation.
A molecule is now defined, and, if the student has not studied physics
sufficiently to become acquainted with the outlines of the kinetic
theory of gases, this theory must be developed sufficiently to give the
student a knowledge of the three great laws of Mariotte, of Charles, and
of Avogadro. He must be made to understand how molecules are defined by
the physicist, and how their relative weights may be inferred by a
comparison of vapor densities. He should then be made to compare the
relative molecular weights, deduced by physical means, with the definite
proportions he has observed in chemical processes. He will thus himself
be led to the conclusion that these definite proportions are the
proportions of the molecular weights, and that the constancy of the law
arises from the fact that in every chemical process the action takes
place between molecules, and that the products of the process are new
molecules, preserving always, of course, their definite relative
weights. The student will thus be brought to the chemical conception of
the molecule as the smallest mass of any substance in which the
qualities inhere, and he will come to regard a chemical process as
always taking place between molecules.
Thus far nothing has been said about the composition of matter. A
chemical process has been defined simply as certain factors yielding
certain products, but nothing has been determined about the relations of
these several substances except in so far as they are defined by the
three laws illustrated above. But now it must be shown that a study of
different chemical processes compels us to conclude that in some cases
two or more substances unite to form a compound, while in other cases a
compound is broken up into simpler parts. Thus, when copper-filings are
heated in the air, it is evident that the material of the copper has
united with that portion of the air we call oxygen to form the black
product we call oxide of copper; and again, when oxide of silver is
heated, it is evident that the resulting silver and oxygen gas were
formerly portions of the material of the oxide. So, when water is
decomposed by electricity, the conditions of the experiment show that
the resulting oxygen and hydrogen gases must have come from the material
of the water, and could have come from nothing else.
Experiments should now be multiplied until the student has a perfectly
clear idea of the nature of the evidence on which our knowledge of the
composition of bodies depends. The decomposition of chlorate of potash
by heat, yielding chloride of potassium and oxygen gas; the
decomposition of nitrate of ammonium by heat, yielding nitrous oxide and
water; the decomposition of this resulting nitrous oxide, when the gas
is passed over heated metallic copper; and, lastly, the decomposition
already referred to, of water by electricity--are all striking
experiments by which the evidence of chemical composition may be
enforced.
The distinction between elementary and compound substances having been
clearly defined by the course of reasoning already given in outline, the
next aim should be to lead the student to comprehend how substances are
analyzed and their composition expressed in percents. The reduction of
oxide of copper by hydrogen gives readily the data for determining the
composition of water, which is thus shown to contain in one hundred
parts 11.11 per cent of hydrogen and 88.89 per cent of oxygen.
Another substance whose analysis can be very readily made by the student
is carbonate of magnesia. By igniting pure carbonate of magnesia in a
crucible (not of course the "magnesia alba" of the shops), the
proportions of carbonic acid and magnesia can be readily determined.
Then, by burning magnesium ribbon, and weighing the product, the student
easily finds the relative weight of magnesium and oxygen in the oxide.
And, lastly, the proportion of carbon and oxygen in carbonic dioxide is
easily deduced from the burning of a weighed amount of carbon. Here the
result may be expressed either in percents of oxide or magnesium and
carbonic dioxide, or else in percents of the elementary substances,
carbon, magnesium, and oxygen.
After making a few analyses like these, the student will be prepared to
comprehend the actual position of the science. All known substances have
been analyzed, and the results tabulated, so that it is unnecessary to
repeat the work except in special cases.
The teacher is now prepared to take a very important step in the
development of the subject. If the molecule is simply a small particle
of a substance in which the qualities of the substance inhere, then it
follows, of course, that the composition of the molecule is the same as
the composition of the substance. The percentage results of the analysis
of water, or of carbonate of magnesia, indicate the composition of a
molecule of water or a molecule of carbonate of magnesia. Thus, 11.11
per cent of every molecule of water consists of hydrogen, while 88.89
per cent consists of oxygen. Hence it follows that, in a chemical
process, the molecules must be divided, and these elementary parts of
molecules which analysis reveals are the atoms of chemistry. Moreover,
as we know the weights of molecules, both by physical and chemical
means, chemical analysis now gives us the weights of the atoms. We have
no time to dwell on the details of this reasoning, but the general
course to be followed will be evident, and it must be enforced by
numerous examples.
Assuming that the student fully comprehends the distinction between
molecules and atoms--that is, between the physically smallest particles
and the chemically smallest particles--he is prepared to master the
symbolical nomenclature of chemistry, with a very few words of
explanation. The initial letters of the Latin names are selected to
represent the atoms of the seventy known elementary substances, and
these letters stand for the definite atomic weights which are tabulated
in all chemical text-books. The symbols of the atoms are simply grouped
together to form the symbols of the molecules of the various
substances; the number of atoms of each kind entering into the
composition of the molecule being indicated by a subscript numeral.
Lastly, in order to represent chemical processes, the symbols of the
molecules of the factors are written on one side and the symbols of the
molecules of the products are written on the other side of an equation,
the number of molecules of each substance involved being indicated by
numerical coefficients.
The atomic symbols, as we have seen, stand for definite weights. In the
same way, the molecular symbols stand for definite weights, which are
the sums of the weights of the atoms of which each consists, and in
every chemical equation the weights of the molecules represented on one
side must necessarily equal the weights of the molecules represented on
the other. The chemical process consists merely in the breaking up of
certain molecules, and the rearrangement of the same constituent atoms
to form new molecules. Again, as the molecular symbols represent
definite weights, the equation also indicates that a definite proportion
by weight is preserved between the several factors and products of the
process represented.
Again, since every molecular symbol represents the same volume when the
substance is in an aeriform condition, it follows that the relative gas
volumes are proportional to the number of molecules of the aeriform
substances involved in the reaction. Thus it is that these chemical
equations or reactions are a constant declaration of the three great
fundamental laws of chemistry.
In order to enforce the above principles, a great number of examples
should now be given which should be so selected as to illustrate
familiar and important chemical processes, including the all-important
phenomena of combustion. In each case, the student, having made the
experiment, should write the equation or reaction which represents the
process, and should be made to solve a sufficient number of
stochio-metrical problems, involving both weights and volumes, to give
him a complete mastery of the subject. Such questions as these will test
the completeness of his knowledge:
Why is the symbol of water H_{2}O? What information does the symbol
CO_{2} give in regard to carbonic-dioxide gas? Write the reaction of
hydrochloric acid on sodic carbonate, and state what information the
equation gives in regard to the process which it represents.
Of course, such questions may be greatly multiplied, and I cite these
three only to call attention to the features of the method of
instruction I have been endeavoring to illustrate.
But, besides teaching the general principles of chemical science, it is
important to give the student a more or less extended knowledge of
chemical facts and processes--especially such as play an important part
in daily life, or in the arts--and such knowledge can readily be given
in this connection. Beyond this I do not deem it desirable to go in an
elementary course of instruction. The way, however, is now opened to the
most advanced fields of the science. A comparison of symbols and
reactions leads at once to the doctrine of quantivalence, and to the
results of modern structural chemistry which this doctrine involves.
Among these results there is of course much that is fanciful, but there
is also a very large substratum of established truth; and if the student
thoroughly comprehends the symbolical language of chemistry, and
understands the facts it actually represents, he will be able to
realize, so far as is now possible, the truths which underlie the
conventional forms.
The study of the structure of molecules naturally leads to the study of
their stability, and of the conditions which determine chemical changes,
and thus opens the recently explored field of thermo-chemistry. To be
able to predict the order and results of possible conditions of
association of materials, or of chemical changes under all
circumstances, is now the highest aim of our science, and we have
already made very considerable progress toward this end.
But I have detained you too long, and I must refer to the "New
Chemistry" for a fuller exposition of this subject. My object has been
gained if I have been able to make clear to you that it is possible to
present the science of chemistry as a systematic body of truths
independent of the mass of details with which the science is usually
encumbered, and make the study a most valuable means of training the
power of inductive reasoning, and thus securing the great end of
scientific culture.