half the sum of the extremes should be equal to the mean; or half 35, the atomic number for chlorine plus half 125, the atomic number for iodine should equal 80, the atomic number for bromine. And this indeed is the result, as will be evident on reference to the following simple arithmetical sum:

"Thus it follows,' to use the beautiful expression of M. Dumas, 'if we could by any means cause the union of halt an atom of chlorine with half an atom of iodine, we might hope to get, to form, to create an atom of bromine !'

"We will not here stop to cavil at the impropriety of assuming the division of an atom, which, according to its derivation, a Tɛμvεiv, cannot be divided. We accept the term as a poetic licence of most innocent description; and as not by any means disturbing the beautiful train of reasoning which M. Dumas has exposed to our gaze. Had the term equivalent been used all verbal objection would have ceased.

"There are floods in the region of philosophic speculation, no less than in the commoner affairs of men. There was once a flood of alchemical aspirations: since then there has been a flood of counter operations so strong, so violent, so overwhelming, that he indeed must be a venturous man who would stand up against it. Very few people would have dared to enunciate the speculations that M. Dumas has propounded with regard to the transmutation of bodies; very few could have done it, for fear of overstepping that little demarcation which stands between the ridiculous and the sublime; but M. Dumas having enunciated his speculations, Dr. Faraday comes forward and says not only has he dreamt and thought, but tried experiments at transmutation, and professes himself so delighted with the doctrines of M. Dumas that he will, if his life be spared, try again.

66

'Leaving the triad of chlorine, bromine, and iodine, M. Dumas next took up for the purpose of investigation a second triad, of sulphur, selenium, and tellurium; bodies which all chemists know to be isomeric or capable of replacing each other n compounds, and to be endowed with properties mutually analogous. Of the three, sulphur is the most volatile, selenium next, tellurium least of all. As to their decomposing power, sulphur replaces selenium; selenium, tellurium; in short, the remarks already applied to the trial of chlorine, bromine, and iodine will apply here.

"Do the generalisations of M. Dumas apply? We will see. "The atomic or combining weight of sulphur is 16; of tellurium 64; the half of the sum of these extremes is the number 40:-and this is the exact atomic weight of the middle term of the triad-the atomic weight of selenium!

"Take, again, the triad calcium, strontium, and barium. Without stopping to indicate the various analogies of these bodies, it will suffice to point out their general chemical similarity. In this scale of analogous qualities calcium and barium are the extremes, strontium is the mean. The atomic weights of the three are as follow:

Calcium, 20.
Strontium, 44.

Barium, 68.

And it will be evident at a glance that there is here a harmony between the chemical qualities and mathematical exponent of their combining proportion as before; for 68 plus 20 divided by 2 is equal to 44, the atomic number of strontium. Thus, if, to use the expression of M. Dumas, by any means we could effect the union of half an atom of barium with half an atom of calcium, we should have as a resultant one atom of strontium!

"Let us take now another triad:

The similarities between the properties of these bodies is too evident to be pointed out; of the three, lithium is the least individualised alkaline metal; potassium the most individualised; sodium, as all know, stands intermediately between the three; and here again, as the most casual examination will demonstrate, the same parity of chemical and mathematical symmetry holds good.

'Now, so extraordinary a symmetry of chemical qualities with mathematical exponents can scarcely be assumed to be a matter of chance; still less can it be said that the atomic figures on which these deductions are based have been strained to suit the opinions of M. Dumas. Before an atomic quantity becomes classical, it is exposed as we all know, to the rigorous scrutiny of many independent chemical tribunals. If, then, exception be taken to the theory of M. Dumas, this exception cannot be allowed to extend to his facts. In theory all may err, and M. Dumas amongst the rest; we confess, however, we are not so paralysed by standing opinions as to fear to think with him.

"Hitherto we have followed the philosopher through his excursion among inorganic triads. We will now attend him further still into the recesses of chemical philosophy. It will be familiar, we doubt not, to most of our readers that many chemists, M. Dumas amongst the number, have regarded certain bodies of compound nature as analogous in many properties to the metals.

"Of this kind are the three organic radicals:

which may be regarded as three several oxides of an isomeric triad, bearing analogy to those already adverted to in the inorganic world. Now, the slightest examination here will prove again that the law hitherto applied holds good in this case. Omitting the oxygen in the three preceding substances, half the sum of the extremes will be equal to the mean. Indeed M. Dumas sees in these manifestations a general law, which may

be thus expressed :—when three bodies having qualities precisely similar, though not identical, are arranged in succession of their chemical powers, there will be also a successive arrangement of mathematical powers, indicated by the respective atomic numbers of the substances; and amenable to every mathematical law.

"That this symmetry of chemical with mathematical function points to the possibility of transmutation is unquestionableyet not transmutation in the sense of the old alchemical philosophy. Chemists see no manifestations of a tendency of being able to convert lead into silver, or silver into gold. These metals are not chemically conformable. One cannot take place of another by substitution. They do not form an isomeric group. The probability is that our first successful transmutation as regards the metals will effect the change of physical state merely without touching chemical composition; thus, already we have carbon, which, as the diamond and as charcoal, manifests two widely different states. Sulphur also assumes two forms, as also does phosphorus. Then, why not a metal? This sort of effect M. Dumas suggests will be amongst our first triumphs in the way of transmutation.

"In conclusion, M. Dumas cited in support of his theory the fact that bodies of conformable qualities were generally found in union or proximity, whether as presented by nature or formed by the agency of man.

"Thus, with iron there is associated manganese. Where nickel exists cobalt is not far off;-and in the organic kingdom, when man elaborates alcohol, there are simultaneously formed small amounts of ethereal bodies. Wherever viewed, chemistry is full of the startling coincidences now introduced to our notice for the first time by M. Dumas; and, whatever may be the difference of opinion as to the speculative notions of the philosopher, there can be no doubt that he has opened a wide store of chemical treasure."]

INTRODUCTION TO LECTURE III.

HYDROGEN-ITS

SYNONYMES AND ETYMOLOGY, HISTORY,

NATURAL HISTORY, PREPARATION, AND QUALITIES.

SYNONYMES. -Hydrogen, idup, water, yɛvvɛiv,

generate. (Lavoisier.)

Inflammable air. (Previous chemists.)
Phlogiston.

to

HISTORY.-Hydrogen was first examined in a pure state by Mr. Cavendish in 1766,* before which time it had been confounded with several of its compounds, under the name of inflammable air.

NATURAL HISTORY.-Hydrogen exists largely diffused in both the inorganic and organic kingdoms. In the inorganic kingdom it is a constituent of various acids in combination-as the hydrochloric, hydrobromic, hydriodic, acids. It is also a constituent of liquids-as water and naphtha; of certain solids—as sal-ammoniac and sulphate of ammonia.

In the organic kingdom hydrogen is found largely

*Phil. Trans. vol. lvi. 144.