have to account for the disengagement of electricity outside the cell in which the chemical changes take place. It cannot be derived from the atoms, for their electrical charges are inseparable, and if it is answered that it is produced by the transformation of chemical force, that is in the plainest manner to admit that chemical force is not electricity, but only a power capable of generating it as heat is. The diffi culties inherent in the electro-chemical theory have been felt by Helmholtz. Thus he says, "I think that the facts leave no doubt that the very mightiest among the chemical forces are of electric origin. The atoms cling to their electric charges, and opposite electric charges cling to each other; but I do not suppose that other molecular forces are excluded working directly from atom to atom. Several

of our leading chemists have lately begun to distinguish two classes of compounds, viz., molecular aggregates, and typical compounds, the latter being united by atomic affinities, the former not."

With regard to the last sentence, I think that the leading English chemists hold an exactly contrary opinion; as in the address to the chemical section at the late meeting of the British Association, Professor Williamson said, alluding to this opinion:-"With respect to these views, it may be noticed that the assumption of combination between molecules as due to some other force than that which binds together the constituents of each molecule-in fact, the assumption of molecular combination as an unknown something different from chemical combination-is open to even more grave objections than those which led us to abandon the dualistic system." In a discussion on a paper by Dr. Odling, that chemist and Dr. Roscoe both supported the views of Professor Williamson.

Helmholtz admits the action of other forces than electrical, although their nature is not even suggested; but the main

agent is still maintained to be electricity. In my opinion, electricity is not even one of such forces, supposing chemical action to be of complex nature, and I found this opinion mainly on, first, the production of free electricity by chemical action, and, secondly, the known action of electricity on compounds, which, so far as ascertained by experiments, is a decomposing rather than a combining agency.

In considering chemical action, two things must be noted. First, in the union of two elements-say chlorine and hydrogen, to adhere to the simplest possible illustration where single atoms of only two elements unite-there is production of heat, and therefore transformation of force or motion. This is the striking fact in combination, and to it attention is naturally attracted, almost to the exclusion of the other phenomena. Second, when two elements have thus combined, there is the force which keeps them joined together, so that when the molecule of hydrochloric acid is reduced to the state of the surrounding matter, so far as heat and electricity are concerned, the component atoms remain joined by a constantly acting force always the same and of enormous strength. I do not know what the figures are with regard to hydrochloric acid, but in water the force which holds the atoms of one milligramme of water together is estimated as equal to force of gravitation acting between masses, which, together, would be 71,300 billions of times the mass of the water.

Now, just as a mass let fall from a great height strikes the earth, producing heat by its arrested motion, and then lies where it fell, pressing downwards with a constant, unchanging force, under the action of gravitation, so the combining atoms, after in some manner developing heat, remain attached to one another. The analogy is suggestive, and the question arises, Is not chemical force one like gravitation? It is not gravitation, its force is far greater; it is not alike for all kinds of

matter, and it only acts at distances almost inconceivably small. The analogies of chemical force with gravitation, however, are so great that it can only be regarded as a specific attractive force, greater in amount as the difference in the properties of the elements is greater. The cause of this attractive force is at present unknown, and I do not pretend to suggest one. The opposing force is heat, and the force of chemical union is the difference between these two forces. Whatever it may be it can hardly be distinguished from the physical force cohesion, as manifested in masses of matter, and the cause of this is equally mysterious. If two surfaces of lead or glass are brought into close approximation, cohesion takes place, and the two masses are so united as to form one whole. Some experiments recently published in the Journal of the Chemical Society, for July, 1881, by Dr. Spring, have greatly extended our knowledge of this force. Powdered bismuth, tin, lead, zinc, copper, &c., by a pressure of from 2,000 to 5,000 atmospheres, became solid homogeneous blocks, having the density of the same metals when cast. Similar results were obtained with sulphur and graphite. With regard to the latter substance, this property of cohering under pressure has long been known, and practically employed in making the blocks of graphite from which black lead pencils are made, and the blocks of inferior quality used for domestic purposes. The most remarkable results, however, are those with differing substances mixed together. Thus, copper filings and sulphur at 500 atmospheres pressure chemically combine, and give sulphide of copper; mercuric chloride and copper, at 5,000 atmospheres, gave copper chloride and mercury; iodide of potassium and mercuric chloride, at 2,000 atmospheres, gave mercuric iodide and chloride of potassium. The abstract does not say if in the first instance the combination of copper and sulphur was attended with heat, as when they combine by heating copper in sulphur vapour,

or if the combination was due to heat developed by the pressure; and we must wait for further researches in this direction for information which bids fair to be of the utmost importance with relation to the formation of chemical compounds. So far as indicated in the abstract, the effect of pressure seems to be to bring the elements within the range of their attractive force, and chemical force then resolves itself into a form of cohesion. Faraday, in his lectures on "The Forces of Nature," held this view, moreover, for he heads the chapters on chemical force with "Chemical

Force-Cohesion."

It may be objected that cohesion takes place between like substances, and chemical force is the strongest between unlikes. But this is not so; cohesion acts powerfully between unlike bodies, and few homogeneous rocks possess the cohesion of heterogeneous granite, whilst in alloys it is impossible to draw a line between simple mixtures of metals and a chemical combination. There is one alloy of zinc and copper, in simple atomic proportion, which possesses definite crystalline form and pure yellow colour, whilst in slightly differing proportions the mass presents the appearance of a simple mixture. Again, iodine and sulphur unite with rise of temperature, and form a crystalline compound, yet alcohol will dissolve out all the iodine. Can this be distinguished from a mixture? And yet, rise of temperature during combination and crystalline form are recognised indications of chemical action. There is indeed an infinite gradation, and no one can say, here cohesion ends and chemical action begins.

This well-known physical force, with regard to the cause of which speculation seems not even to have exercised itself, as I have not met with any theory regarding it, may not seem sufficient to account for the phenomena of chemical action; but when we see the force of cohesion between molecules, as seen in steel, a wire of which one square

millimetre in sectional area will bear a weight of 184 lbs., or in the diamond, whose particles are almost indissolubly bound to one another, it is easy to imagine that in the closer contact of atomic cohesion a force may be exerted quite equal to explain all the stability met with in compounds. The difference in properties between a compound and those of the elements of which it is composed may seem too great to be explained by the mere contact of the atoms. But all the properties of matter appear to depend on molecular arrangement; and seeing the total alteration of the properties of phosphorus when transformed from the yellow into the red modification-a change embracing not only physical, but even chemical properties; when colour, hardness, transparency, specific gravity, and even solubility, oxidizability, and action on the animal economy, are all changed, we cease to wonder at the change in properties when a new molecule is formed with two or more differing elements with differing properties.

As with molecular cohesion, so with atomic: heat is the opposing agent. The tenacity of metals diminishes as the temperature rises, until the solid is resolved into a liquid, and the liquid finally into a gas. So with chemical compounds; some can only exist at the lowest temperatures which we can produce, others require but a touch before the bonds of cohesion are destroyed, and the elements go off beyond the bounds of their mutual attraction. As we raise the temperature, more and more chemical compounds are disassociated, some splitting into simpler compounds, but at last separating into their elements, until in heat far surpassing any of man's device, in the solar furnace, where heat beyond the imagination of man struggles with atomic cohesion, compounds such as we know them seem to be impossible, and we seem to behold indications of the decomposition of what we call elements.

The decomposition by