CH. XXXV.

THE ELECTRIC TELEGRAPH.

359

a great reservoir, as it were, of electricity, so that when the current runs into it at one place an equal amount must run out at another; but all that is really known is that the whole ⚫ globe acts practically as a return wire.

3. The magnetic needle is made of two or more parts, for since it would be very inconvenient if the pointer were always trying to turn to the north, this is avoided by fastening two needles side by side, with the north pole of the one lying

Diagrams showing the general principle of the Electric Telegraph.

A, A', Galvanometer, or box containing the magnetic needle. B, B', Commutator, or box in which the telegraph wire and earth wire are joined to each other as in B', or to the battery, as in B. c, d, Telegraph wire. e, Earth wire. fg, Copper plates at the end of the earth wire. The arrows show the direction of the positive

current.

against the south pole of the other, and thus, as the earth attracts each needle in a different way, the pull is neutralized. This double needle is called an astatic needle, and it is so placed in the box A in the form of telegraph we are describing, that one needle is inside surrounded by the wire, while the other is outside on the face of the box.

YORK

4. The commutator, B, is a box with an apparatus inside which is so arranged that by turning a handle (not shown in the diagram) different ways the earth wire and telegraph wire can be joined together, or either of them can be joined to one of the poles of the battery.

The commutator and galvanometer are really made in one instrument, but I have drawn them separate to make it more clear.

Now, when the man in London wants to send his message to York, he first sends off a current which rings a little bell at all the stations along the line to call attention, and then spells out the word York. This warns the man at that station to turn the handle of his commutator, B', so that the telegraph-wire, d, and the earth-wire, g, are joined together. Then the message can be sent. The man in London turns his handle according as he wishes the current to go. In Fig. 60 he has turned it so that the telegraph wire, c, is joined to the positive pole of the battery, and the current will pass above ground along c d to the galvanometer A', turning the needle to the right, and will then go back through the earth by gfe to the battery. But in Fig. 61 the man has altered the handle, and now the earth wire, e, is joined to the positive pole, and so the current passes underground at e f, and out at g, and entering the galvanometer on the left side, turns the needle to the left, and goes back by the telegraph wire, dc, to the battery. In this way he turns it from right to left as he will, and spells out the message thus: Left, right/ = A; left, right, left, left,,|,, = L; left, right, right // W; left, |= E; therefore; ; ; ; 、; \/\ ; \/\ ; spells 'all well!'

= :

It is not necessary to have a separate wire for every telegraphic station: one wire will do all the work so long as it is

CH. XXXV.

THE AMERICAN TELEGRAPH.

361

only used by one man at a time. Therefore at every station there is a galvanometer to point out the message, a battery to provide the current, and a commutator to change the current; but these are not joined to the general wire unless they are being used. In Morse's American telegraph, which is generally used on the Continent, the needle pricks holes in a strip of paper, so that the message can be kept, and Bain's electro-chemical telegraph writes down the marks on chemical paper. But all these are only improvements of the same principle by which an electric current going first one way and then another acts on a magnetic needle.

Chief Works consulted. Lardner's Cyclopædia - ' Electricity, Magnetism, and Meteorology;' 'Annals of Philosophy,' New Series, 1822, vols. ii. and iii.; History of Magnetism;' 'Encyclopædia Metropolitana,' art. 'Electro-Magnetism ;' Faraday's 'Experimental Researches in Electricity,' 1859; Tyndall's 'Faraday as a Discoverer ; ' Gladstone's 'Michael Faraday ;' 'Nouvelle Biog. Universelle'—' Ampère,' 'Oersted'; Ampère, 'Observations Electro-dynamiques,' 1822; Faraday, Various Forces of Nature;' Proctor, The Sun ;' Herschel's 'Familiar Lectures;' Brande's 'Manual of Chemistry.'

17

CHAPTER XXXVI.

SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). Davy discovers that Nitrous Oxide produces Insensibility—Laughinggas-Safety-lamp, 1815-Nicholson and Carlisle discover Decomposition of Water, 1800–Davy discovers the effect of Electricity upon Chemical Affinity-Faraday's Discoveries in ElectrolysisIndestructibility of Force-Various modes discovered of Decomposing Substances-John Dalton, Chemist-Law of Definite Proportions-Law of Multiple Proportions-Dalton's Atomic TheoryThe study of Organic Chemistry-Liebig the great Teacher in Organic Chemistry.

Sir Humphry Davy, 1778-1829.-We saw in the last chapter how Oersted, Davy, Ampère, Faraday, and Seebeck, by their various discoveries, showed the connection between Electricity, Magnetism, and Heat. We must now learn how the connection between electricity and chemical change was also worked out. This was done by Sir Humphry Davy and Faraday, who thus put England once more at the head of chemical discovery, in which the French school of Lavoisier had so long taken the lead.

Sir Humphry Davy, whom we have mentioned before as making experiments upon heat, was born in 1778, at Penzance, in Cornwall, and died at Geneva in 1829. His mother being a widow, he was apprenticed when quite young to an apothecary, and there with wine glasses, old medicine bottles, tobacco pipes, and a syringe, he made his first chemical experiments. When he was scarcely twenty years

CH. XXXVI.

SIR HUMPHREY DAVY.

363

of age, Dr. Beddoes, a physician, who had opened a hospital for curing patients by the use of different gases, heard so much of the young man's abilities that he invited him to come to Bristol, where he employed him in making experi

ments.

In this way Davy's attention was drawn to nitrous oxide, a gas which had been declared by a celebrated physician, Dr. Mitchell, to be very poisonous. Our young chemist wanted to try this for himself, and actually began breathing it in small quantities to see whether it would affect him. He proved that it certainly was not so poisonous as Mitchell had thought, and, growing gradually bolder and bolder in the use of it, he succeeded at last in breathing the gas for several minutes, at the end of which time he lost all consciousness, and found himself in a land of delicious dreams, out of which he awoke gradually without being injured in any way. Enchanted at having discovered such a delightful sensation, he carried on his experiments for more than ten months, and when he published the results, and told the world that the mere breathing of a gas could make a man sleep, and dream, and laugh without any cause, it created a great sensation, and Davy's name soon became well known.

At this time (1801) the Royal Institution had just been founded, and Count Rumford, seeing that Davy was a young man of great talent, offered him the appointment of Assistantchemist. Davy accepted it, and from that time devoted himself entirely to science. He was young, bright, and enthusiastic, and his lectures were so clear and eloquent, that the Royal Institution soon became famous under his influence, while every new appliance for making chemical experiments was given him' in his laboratory. It was here that he made his observations on flame in 1815, and constructed his Safety