144 



NATURE 



[y?inc 20, 1872 



Recollecting that no physical theory of the constitution 

 of matter had yet been advanced which thoroughly con- 

 formed to chemical ideas, he insisted upon the advisability 

 in teaching the molecular and atomic theory, to keep it 

 free from all that is not absolutely essential, so that it may 

 preserve tufficient plasticity to adapt itself to the progress 

 of our physical and mathematical knowledge. 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 rolions of atoms and molecules by 

 the comparison of the composition of equal volumes of 

 the bodies in ihc gaseous state, it becomes necessary to 

 consider the difficulties which arise in the application of 

 these notions when the vapour densities aie 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 Avogadio 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 formula: of all compounds as 

 of equal probability. "It is not by concealing the 

 obscurity of these questions that we shall enlighten the 

 student ; on the contiaiy, 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 introducticn, 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 mole- 

 cules has led chemists to the law of substitution, to the- 

 theory of types of Dumas, then to that of Williamson 

 and Gcrhaidt, 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, 

 cntiiely to eliminate this latter theory, which gives a 

 summary of several laws, and guides us ordinarily 

 in the co-ordination 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 doc- 

 trine and the actual phase of development which 

 it has at present reached. It is still far from being a ccm- 

 plete and well-established theory, but is in a state of 

 transition ; for although doubtless it embraces a large 

 number of facts, as jet 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 relation 

 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 v.alencyof atoms as a pro- 

 perty inherent in them, and then to deduce as a corollary 

 their different modes of union ; on the contrary, it is pre- 

 ferable to regard each portion of this doctrine as a deduc- 

 tion from the observation and comparison of a deter- 

 minate group of facts, until an opportunity offers to unite 

 these fragments into one whole, not forgetting, however, 

 to notice the gapg which exist, never going beyond what 

 the I'acts themselves suggest, and never applying 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 

 hcxa-valent, others arc tri- and pcnta-valent ; but the pupil 



should be prevented from acquiring mechanical and geo- 

 metrical 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, " relative position of atoms in the 

 molecules," and even to represent them graphically, we 

 must warn the student that these are only artifices to ex- 

 press certain transformations, and that we are really igno- 

 rant of the relative] position of the atoms either in space 

 cr in the mutual [action of different portions of matter. 

 With these reservations, it is possible, in^teaching to de- 

 rive considerable advantage from] the theory of atomicity 

 and at the same time to avoid its inconveniences. 



In the study of the transformations which matter under- 

 goes, we should direct the pupil's attention, not only to the 

 ponderable changes in the compositionof molecules, buta!so 

 to the electrical and calorific phenomena which accom 

 pany these transformations. Even from Lavoisier's time 

 it has been recognised that we cannot separate the study 

 of matter from thermic considerations; and everyday the 

 connection which exists between chemical and thermic 

 phenomena becomes more apparent. 



As in the study of ponderable changes we were guided 

 by the law of the conservation of weight, so in the con- 

 nection between chemical and dynamical phenomena we 

 are guided by the law of the conservation of force ; the 

 two studies mutually supplementing and illustrating one 

 another. Not only will the atomic and molecular theory 

 and that of atomicity help us to compare dynamical phe- 

 nomena, but the study of dynamical phenomena will show 

 us analogies and differences between chemical actions 

 which would not be observed in the ponderable equations. 

 We should therefore instruct the student in the little defi- 

 nite knowledge which we at present possess concerning 

 thermic and electric phenomena, and especially fix in his 

 mind the fundamental notion of a mechanical equivalent, 

 and the manner of comparing it with chemical action as 

 expressed by the atomic theory. In this we should be 

 aided by the previous or simultaneous instruction of the 

 student in physics under the form and language of the 

 themiodynamic theory. 



The lecturer concluded by observing that in the choice 

 of methods and of matter for a course of chemistry, it 

 should always be borne in mind that it was eminently a 

 progressive science, and that even at the time of its most 

 rapid development. The student should start not only 

 wiih a knowledge of certain definite and fixed principles, 

 but with an aptitude and sufficient preparation to enable 

 him to follow the science in its unceasing transformation 

 and progress, whether he intends expressly to cultivate 

 chemistry, or has only learnt the elements of the science 

 as an auxiliary to other studies or professions. MoiC- 

 over, the end of chemical instruction for both these classes 

 of students is not only to fix in their memory a certain 

 amount of knowledge, but to assist in their intellectual 

 education. For this, chemistry of all sciences is one of 

 the best, offering both in verbal and piactical instruction — 

 excellent occasions for the exercise and harmonious de- 

 velopment of all the faculties of the human mind. 



He had desired to call attention to what he consideied 

 to be the most efficient means of imparting a knowledge 

 of chemistry so that it might serve as an instrument of 

 intellectual education, and that the student, by following it 

 in its ulterior developments, might judiciously apply it to 

 the study of the other branches ot natural science. If 

 the attention of the eminent chemists and professors there 

 present were once attracted to this subject, he felt certain 

 that a bright light would be thrown on it, and that our 

 young professois would find numerous suggestions to 

 direct them in teaching chemistry, and that at the very 

 moment when instruction in our science had become so 

 difficult, on account of the rapid transformation which it 

 was uneiergoing. 



