536 



NATURE 



[OcL 6, 1887 



oppositely charged {i.e. the walls encroaching on the cavity), 

 the far side similarly charged (the cavity encroaching on 

 the walls), and the pressure on the fluid in the cavity being 

 increased or diminished in correspondence with the 

 change of pressure in the charged or inducing cavity. In 

 other words, conductors rise in potential when brought 

 near a positively charged body. 



The actual changes in volume necessary to the strain 

 of these cavities are a defect in the analogy. To avoid 

 this objection, one will have to accept a dual view of 

 electricity — a sort of two-fluid theory, which many pheno- 

 mena urge one to accept, but about which I will say 

 nothing to-night. It is sufficient at first to grasp the one- 

 fluid ideas. 



Return Circuit. — Sometimes a difficulty is felt about 

 electricity flowing in a closed circuit — as, for instance, in 

 signalling to America and using the earth as a return 

 circuit : the question arises. How does the electricity find 

 its way back ? 



The difficulty is no more real than if a tube were laid 

 to America with its two ends connected to the sea and 

 already quite full. If now a little more sea-water were 

 pumped in at one end, an equal quantity would leave the 

 other end, and the disturbed level of the ocean would 

 readjust itself. Not the same identical water would 

 return, but an equal quantity would return. That is all 

 one says in electricity. One cannot label and identify 

 electricity. 



To imitate the inductive retardation of cables, the tube 

 should have slightly elastic walls ; to imitate the speed of 

 signalling, the water must be supposed quite incom- 

 pressible, not elastic as it really is, or each pulse would 

 take three-quarters of an hour to go. 



Condensers. — Returning to the subject of chargirg 

 bodies electrically, how is one to consider the fact that 

 bringing an earth-plate near a conductor increases its 

 capacity so greatly, enabling the same pressure to force 

 in a much larger quantity of fluid ? how is one to think of 

 a condenser, or Leyden jar? 



In the easiest possible way, by observing that the 

 bringing near an earth-connected conductor is really 

 thinning down the dielectric on all sides of the body. 



The thin -walled elastic medium of course takes less 

 force to distend it a given amount than a thick massof 

 the same stuff took. A Leyden jar is like a cavity with 

 quite thin walls — in other words, it is like an elastic bag. 

 But if you thin it too far, or strain it too much, the 

 elastic membrane may burst : exactly, and this is the dis- 

 ruptive discharge of a jar, and is accompanied by a 

 spark. Sometimes it is the solid dielectric which breaks 

 down permanently. Ordinarily it is merely the air ; and, 

 since a fluid insulator constitutes a self-mending partition, 

 it is instantaneously as good as new again. 



There are many things of interest and importance to 

 study about a Leyden jar. There is the fact that if insu- 

 lated, it will not charge : the potential of both inner 

 and outer coatings rises equally ; that, in order to charge 

 it, for every positive spark you give to the interior an 

 equal positive spark must be taken from the exterior. 

 There is the charging and the discharging of it by 

 alternate contacts, as by an oscillating ball ; and there are 

 the phenomena of the spark-discharge itself. 



But, as you know, all charging is really a case of a 

 Leyden jar. The outer coat must always be somewhere 

 — the walls of the room, or the earth, or something — you 

 always have a layer of dielectric between two charges — 

 the so-called induced and the inducing charge. You 

 cannot charge one body alone. 



To illustrate the phenomena of charge, I will now call 

 your attention to these diagrams — which less completely 

 but more simply than hydraulic illustrations, serve to 

 make the nature of the phenomena manifest. 



^0 be continued.) 



ON THE TEACHING OF CHEMISTRY.^ 



THE question is being often asked, Why does chemis- 

 try progress so slowly in this country ? Different 

 answers, all more or less true, may be given ; one answer 

 that has not, I think, been sufficiently insisted on is: 

 Because chemistry is so little taught. 



Classes, nominally in chemistry, are conducted in many 

 schools, and in almost all the colleges, of the country ; 

 but I assert that very little of what is taught is really 

 chemistry. For what is it that is taught ? On the one 

 hand, catalogues of so-called facts detached from reason- 

 ing and from generalizations; on the other hand, definitions 

 and generalizations and speculations detached from the 

 facts on which they rest. But neither detached facts, 

 however accurately stated, nor definitions, in however 

 sharply-cut words they are contained, nor speculations, 

 however interesting they may be, are science ; and 

 chemistry really is a branch of natural science. 



It is admitted by all that hydrogen is a colourless, 

 odourless gas, 14-435 times lighter than air, produced by 

 the interaction of dilute sulphuric acid and zinc, com- 

 bustible, condensable to a liquid at very low tempera- 

 tures. These statements are facts ; but when the student 

 of chemistry is required to read, and if possible to re- 

 member, such facts as these about each of the elements 

 and its compounds, the statements cease to be facts to 

 him, and become false, inasmuch as they cover up and 

 hide the really important facts regarding the interactions 

 of elements and compounds, and regarding the con- 

 nexions between changes of composition and changes of 

 properties, which form the subject-matter of chemistry. 



I have known students have at their finger-ends the 

 properties of all the elements— as these properties are 

 detailed in the ordinary text-books— and yet be almost 

 wholly ignorant of chemistry. , 



And I have also known students ready at a moment s 

 notice to repeat the orthodox definitions of atomic and 

 molecular weights, or to draw structural formulae of com- 

 plex minerals, or to speak fluently about double bonds 

 and unsaturated units of affinity, and yet be quite innocent 

 of any knowledge of chemistry. 



A fatal distinction is too often drawn by chemical 

 teachers between the facts on which chemical science 

 rests, and reasoning and generalizing on these facts : the 

 former, that is statements of detached facts, is called 

 chemistry ; the latter, that is reasoning on facts and 

 generahzing to principles, is called chemical philosophy. 

 I believe strongly that there are not two chemistries, but 

 one chemistry. If chemical teachers were quite decided 

 as to what they ought to teach, we might hope for marked 

 advances in our science. 



My own experience in teaching chemistry convinces me 

 that it is a very difficult subject both to teach and to learn. 

 Of course there is no great difficulty in restating to a class 

 what is printed in the text-book, and occasionally en- 

 livening the routine by a few experiments ; nor does it 

 require high mental capacities and training to tell the 

 laboratory student that bottle A contains a double salt, 

 to be analyzed by the help of the tables on page so-and-so of 

 the book. But do not let us call this kind of thing teaching 

 chemistry. To teach chemistry well requires experience 

 and an educated mind. It is not easy to hit the golden 

 mean. If the teacher despises facts his reasoning be- 

 comes absurd, because it is based on nothing ; his princi- 

 ples become only speculations ; and his laws merely 

 phrases. If he disregards principles, generalizations, and 

 theories, his facts become false, and when facts are false 

 (as they often are) they are deadly, and kill those that 



trust in them. . , r -tt. 



Chemistry is a branch of natural science ; it deals witn 



one class of natural occurrences, it observes and expen- 



' A Paper read before Section B of the British Association at Manchester, 

 by M. M. Pattison Mulr, M.A., Fellow of Gonville and Cams College, 

 Cambridge. 



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