hence the chlorine is regarded as un-equivalent or univalent, and as the chlorine is merely the type of a class, it is stated to belong to the monohydric group of elements. On the other hand, oxygen (with its new combining proportion, 16 instead of 8), always combines with hydrogen in the proportion of one volume of oxygen to two volumes of hydrogen, forming two of steam; and when these two volumes of steam are separated again into oxygen and hydrogen, they are resolved into one volume of oxygen and two volumes of hydrogen. Oxygen, therefore, unites with two proportions of hydrogen; and it is said that oxygen and its class are bi-equivalent or bivalent, and the group is named the dihydric class.

Again, nitrogen unites with hydrogen in the proportion of one volume of nitrogen with three volumes of hydrogen, forming two volumes of gaseous ammonia, from which one volume of nitrogen and three volumes of hydrogen can be separated. The nitrogen, therefore, is tri-equivalent or trivalent, and the class of which nitrogen is the type is the trihydric group. And, lastly, carbon (with its new combining proportion of 12, instead of 6) when it combines with hydrogen, does so in the proportion of one volume of carbon (theoretical vapour) with four volumes of hydrogen, forming two volumes of marsh gas, from which can be separated the one volume of carbon and the four volumes of hydrogen. Carbon and its class always combine with four of hydrogen, and are therefore quadrequivalent or quadrivalent, and the class of substances, of which carbon is the type, is called the tetrahydric group. These classes of bodies may be summed up in the following statement,-one volume of chlorine is satisfied with one volume of hydrogen; one volume of oxygen demands two volumes of hydrogen; one volume of nitrogen calls for three volumes of hydrogen, and one volume of carbon is not content until it receives four volumes of hydrogen.

In the construction of the various organic substances, the relative satisfying powers of the chlorine, oxygen, nitrogen, and carbon atoms must be taken into account; and if the carbon does not obtain its four volumes of hydrogen, or its

quadruple power of combining be not satisfied, then an imperfect or unstable compound is formed. Several atoms of carbon may be associated together, and, indeed, compounds of the alcohol series are known as high as thirty atoms of carbon, and all these must be satisfied at four points, by being in union with hydrogen or the neighbour atoms of carbon. Where a vacant space or point of the carbon occurs, then it represents an unfinished or unsatisfied molecule.

The law of substitution in organic substances enables the chemist to vary the compounds which are produced. Thus, whilst in marsh gas the one of carbon unites with four of hydrogen, there is no difficulty in taking the compound thus formed and abstracting one, two, three, or even all the four atoms of hydrogen, and putting in one, two, three, or four of chlorine instead; so that there can be formed, compounds of one of carbon with three of hydrogen and one of chlorine, with two of hydrogen and two of chlorine, with one of hydrogen and three of chlorine, and, lastly, with four of chlorine alone. In this manner many new substances can be formed; and instead of the chlorine, other elements may be employed in substitution, such as bromine and iodine, forming a new series of substances.

The researches in organic chemistry have led to the actual construction of complex organic compounds from the simplest elements. Till within the last few years, organic compounds were regarded as being the property only of the vegetable or animal kingdoms; and it was believed that when the chemist desired to possess any of the members of the group, he must have recourse to the plant or the animal, and crave from it a part of its substance. Now-a-days, however, the organic compounds are manufactured by the chemist without the aid or intervention of the plant or the animal, and every year adds to the number of substances thus produced. Among the organic compounds which have thus been artificially formed are urea, glycerine, grape sugar, alcohol, and prussic acid.

The advances which have been made in organic chemistry in modern times teach us, that by merely following the

beaten tract of observation, by simply travelling the roads already laid down, there are apparently many millions of substances waiting to be formed and named. Indeed, it has been calculated that there are several sextillions of organic bodies which may yet be discovered by carrying out the present modes of research.

The progress of the science of chemistry, where the elements are combined together to form compounds, may be likened to the union of letters to form words; but the process does not stop there, but may be still further considered to be the throwing of these words into sentences, forming an endless chain of ideas-an endless chain of new chemical compounds. It should, therefore, be observed that the various substances which form types of the classes of bodies are representatives of hundreds of substances which are already known, and doubtless of millions of bodies to be yet formed. And remember that twenty years ago this country was undiscovered; the roads or modes of operating were few; but each year must add to the number of new substances as the great domain of organic chemistry is explored.

And how much may we expect to reap as the reward of human labour in chemical science? Natural substances, as found in the mineral, vegetable, or animal kingdoms, are few as compared with the compounds which are artificially formed, and yet not artificially. The ingenuity of man is exerted on natural substances by natural laws, and he as the artificer merely places stone upon stone, and chemical affinity or attraction binds them together.

How many more substances of vast commercial interest, such as the aniline colours, may be compounded by man, it would be hard to tell; and how many more of medical interest, such as chloroform, it would be presumptuous to speculate upon. As far as the locomotive or iron horse surpasses all the energies of the animal as an aid in locomotion, and gunpowder in the rifle eclipses the human arm in the range and power of its aim, so, doubtless, the researches and applications of chemistry will continue to transform natural substances into new compounds of vast interest to humanity.

VOL. VII.

Suggestions for a Proposed Uniform System of Fog Signals. By ALEXANDER CUNINGHAM, F.R.S.S.A., Secretary to the Commissioners of Northern Lighthouses.*

Since I had the honour of reading a paper on "Fogs at Sea and Fog Signals on Shore" (9th February 1863), various proceedings have taken place, and, although slowly, we seem to be progressing surely in the direction of obtaining at least some improvement on the system, or rather want of system, which then prevailed.

Before proceeding to the subject more immediately under consideration, I am desirous of correcting an oversight in my former paper. It was stated that all that was to be found in the account of the building of the Bell Rock were the two incidents then given; but that statement was not correct. There are several notices of fog in that work, and, among them, the following remarkable passage which had escaped me:

"The boats landed this evening, when the artificers had again two hours' work. The weather still continuing very thick and foggy, more difficulty was experienced in getting on board of the vessels to-night than had occurred on any previous occasion, owing to a light breeze of wind, which carried the sound of the bell, and the other signals made on board of the vessels, away from the rock. Having fortunately made out the position of the sloop 'Smeaton,' at the N. E. buoy, to which we were much assisted by the barking of the ship's dog, we parted with the 'Smeaton's' boat, when the boats of the tender took a fresh departure for that vessel, which lay about half-a-mile to the south-westward. Yet such is the very deceiving state of the tides, that although there was a small binnacle and compass in the landingmaster's boat, we had, nevertheless, passed the 'Sir Joseph' a good way when, fortunately, one of the sailors catched the sound of a blowing-horn. The only firearms on board were a pair of swivels of one-inch calibre; but it is quite surpris

*Read before the Society on 26th March 1866, and awarded the Society's Silver Medal.

ing how much the sound is lost in foggy weather, as the report was heard but at a very short distance. The sound, from the explosion of gunpowder, is so instantaneous that the effect of the small guns was not so good as either the blowing of a horn or the tolling of a bell, which afforded a more constant and steady direction for the pilot. It may here be noticed that larger guns would have answered better; but these must have induced the keeping of a greater stock of gunpowder, which, in a service of this kind, must have been attended with risk. A better signal would have been a bugle horn, the tremulous sound of which produces a more powerful effect in fog than the less sonorous and more sudden report of ordnance."

What renders this passage remarkable is, that the late Mr Stevenson seems to have foreseen the very means which are now recommended, of giving fog signals by horns, and points out the objections to the use of guns.

Having alluded to this matter, I now proceed to the subject in hand.

In the previous paper a hope was expressed that the statements in it might suggest to some more fertile mind a scheme capable of being reduced to practice; but no such scheme has been received or promulgated, and I have not been able to devise anything which satisfies my own wishes on the subject, though it may be hoped that the suggestions to be submitted in this paper may be considered a movement in the right direction.

Some time ago I proposed to a naval friend the following scheme-the great object to be attained being the establishment of some distinctive signal along the coast, which should convey to the ear of the mariner in fog the same information which the distinctive character of a light conveys to the eye in clear weather. I suggested,

1. That one species of distinctive sound should be selected for the main coast where sea lights are exhibited; a second for bays, inlets, or narrow sounds; and a third for harbour mouths.

2. For the first, the main coast, I found it already divided into convenient and well-known spaces for longitude and