the contrary, olefiant gas to the action of chlorine, we find that the chlorine is fixed directly, without substitution, the chlorine atoms meet,

so to speak, with vacant spaces existing in the olefiant gas molecule; in order to get in, they need not expel a corresponding number of hydrogen atoms to make room for them. The compound generated is the so-called Dutch liquid, an oily substance first produced by an association of Dutch chemists at the close of the last century. It was the production of this oily liquid that gave rise to the name of olefiant gas. The number of chlorine atoms thus received directly without substitution is two, corresponding exactly with the number of attraction units that remained unsaturated. Any further number of chlorine atoms are found to enter by substitution, and by substitution only. Similar phenomena are observed when olefiant gas is brought into the presence of bromine. We have here a large glass vessel containing some bromine and water; the vessel, by means of a flexible tube, is connected with a gasholder filled with olefiant gas. On agitation, we see the olefiant gas rushing into the vessel as into a vacuum. The olefiant gas fixes two atoms of bromine, being converted into a transparent colourless liquid, the substance called dibromide of olefiant gas. Here again the combination takes place without substitution.

The behaviour of olefiant gas, under the influence of chlorine and bromine, elucidates the nature of its molecule. The facility with which this gas is capable of fixing two atoms of chlorine to become Dutch liquid, two atoms of bromine to become bromide of olefiant gas, and by roundabout processes two atoms of hydrogen to become hydride of ethyl-all three finished molecules-characterizes olefiant gas as a molecule interrupted in its growth, and in which the power of resuming this growth, and the limit of its final development, may

be traced by the simplest experiments. The apparently anomalous construction of the olefiant gas molecule is thus most satisfactorily accounted for. Indeed, far from disturbing the harmony of the rules of combination elicited by our inquiries, a closer examination into the nature of this compound, whilst explaining whatever appeared exceptional in its construction, leads us, on the contrary, to a loftier interpretation of these rules, to the conception of compounds, the very structure of which foreshadows the more prominent features of their chemical character.

I have selected olefiant gas as an example of a class. We remember that this substance is the first term of a long list of homologous bodies, in all of which we find similar structure combined with similar chemical properties. All these substances, and, let me add, a great variety of others, we have to regard as molecules arrested under special circumstances at a certain stage of their development, but capable, under favourable conditions, of growing again, until by the perfect balance of the atomic attractions within, they have ultimately arrived at maturity.

We have thus been led, step by step, to a distinction of a novel kind, that of finished and unfinished molecules; or, to use the more frequently employed expression, that of saturated and non-saturated compounds. I need not tell you that this distinction carries us to the threshold of a new field of research, hitherto crossed only by a small band of fearless pioneers, who are encountering difficulties on all sides. Admitting, as we are compelled to do, the existence of what we have called unfinished molecules, we inquire under what special conditions, at what special stages the growth of a molecule may be arrested? How is it that as yet the marsh-gas molecule is known only in the finished state, CH, that none of the fragmentary marsh-gases, CH, CH,, and CH,, which might exist, have ever been obtained? Again, how is it that the molecule of hydride of ethyl exists, so to speak, finished and unfinished; and, lastly, that of the several fragmentary states in which this molecule might be met with, two only, namely the two states, C, H. (olefiant gas), and C, H, (acetylene), have ever been observed? We are thus brought face to face with some of the most deeply interesting problems of chemical mechanics, in the solution of which the exertions of chemists are engaged at the present moment. I must not, however, dwell upon the interest attached to this new line of inquiry, upon the numerous experiments which the idea of saturated and nonsaturated compounds has already suggested, and on the influence it is likely to exercise on the direction of chemical investigation for some time to come.

Nor am I permitted to follow these speculations into another direction. I have to forego, more especially, the pleasure of submitting to you some of the ingenious explanations which Professor Kekulé, to whom we are greatly indebted for the development of this branch of chemistry, has advanced for the elucidation even of saturated com

pounds of anomalous constitution. Tempting though the further elaboration of this subject may appear, it would lead me inevitably beyond the legitimate limits of a Friday evening lecture at the Royal Institution.

Indeed my time, and, I fear, your patience, are exhausted, and I must add but few concluding words. Your attention so kindly bestowed on my remarks will not, I trust, have been entirely thrown away, if I have succeeded in convincing you that modern chemistry is not, as it has so long appeared, an ever-growing accumulation of isolated facts, as impossible for a single intellect to co-ordinate as for a single memory to grasp.

The intricate formulæ that hang upon these walls, and the boundless variety of phenomena they illustrate, are beginning to be for us as a labyrinth once impassable, but to which we have at length discovered the clue. A sense of mastery and power succeeds in our minds to the sort of weary despair with which we at first contemplated their formidable array. For now, by the aid of a few general principles, we find ourselves able to unravel the complexities of these formulæ, to marshal the compounds which they represent in orderly series; nay, even to multiply their numbers at our will, and in a great measure to forecast their nature ere we have called them into existence. It is the great movement of modern chemistry that we have thus, for an hour, seen passing before us. It is a movement as of light spreading itself over a waste of obscurity, as of law diffusing order throughout a wilderness of confusion, and there is surely in its contemplation some thing of the pleasure which attends the spectacle of a beautiful daybreak, something of the grandeur belonging to the conception of a world created out of chaos.

[A. W. H.]

WEEKLY EVENING MEETING,

Friday, April 28, 1865.*

SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. Vice-President, in the Chair.

DR. LYON PLAYFAIR, C.B. F.R.S.

On the Food of Man in relation to his Useful Work.

THIS discourse was in three divisions. The first division treated of the amount of food required for mere subsistence; then for the full health of the non-labouring adult; and lastly, of the quantity necessary for an active labourer. The second division of the discourse discussed the question whether there was sufficient potential energy in the nitrogenous tissues, and in the oxygen required for their transformation, to account for the dynamical actions within or without the body. The question as to whether the fatty and amylaceous ingredients of the food co-operated in this work was brought under review. The third division of the discourse treated of the secretions per vesicam and per anum as measures of work.

In the first division of the discourse a number of subsistence and low dietaries were recorded, and, as a general average, the following diets were given in ounces of 437 grains :

The speaker then examined the food of soldiers during peace as giving a fair average of food required by adult men, of soldiers engaged in work like the Royal Engineers, and of those exposed to the fatigues of war, as giving diets necessary for labourers. The following averages were given in ounces :

This discourse has since been published in extenso by Edmonston & Douglas, Edinburgh.

Active labour was defined to consist of work which would enable a man to walk twenty miles every day throughout the year, except on Sundays. The labour during war is much the same, for soldiers marching fourteen miles daily, with 60 lbs. weight of accoutrements, exercise labour amounting to 776,160 foot pounds, while the pedestrian walking twenty miles exerts a force of 792,000 foot pounds.

In the second division of the discourse, the speaker showed that the common experience of mankind is in favour of the nitrogenous ingredients of food being the source of dynamical work. Horses and men, when labouring, are provided with food rich in such substances, and their labour was shown by numerical data to be proportional to the amount of the former. Thus the work of a horse, divided by the work of an ox, gives the ratio 1: 143, while the plastic food of these animals, treated in the same way, yields 1: 1·44. In the same way, the work of a horse is eight times greater than that of a man, and the plastic food used for the external dynamical labour of each is nearly in the same proportion. The equation of decomposition used by the author is the following one :

Albumen.

=

Ca. Has No 01 + 100 O 3 (CO, (NH2)2) + 21 CO, + 13 (H, O1)

4

16

In this equation, the small quantity of sulphur in albumen is viewed as oxygen. The simplicity of the equation is remarkable; for of the two forms of carbonic acid produced, the one, amido-carbonic acid, passes away per vesicam, and gaseous carbonic acid per halitem. Seven times as much carbon should appear in the latter as in the former secretion, and this is exactly what has been found in the case of dogs fed with flesh free from fat. Using Andrew's units of heat and the above equation, one ounce of transformed tissue (28.35 grammes) would raise 126.5 kilogrammes of water, 1° C., or converted into its mechanical equivalent by Joule's number 425, would raise 53,762 kilogrammes one metre high.* These numbers are easily applied. Soldiers during peace are well exercised by a march of seven miles daily. Their useful external work is therefore 38,333 metre kilogrammes; while the potential energy in the 3.94 oz. of flesh-formers (remaining after deducting the amount in the alvine evacuations) is 211,822 metre kilogrammes. But the internal dynamical work of the heart, respiratory and other movements, require 107,524 metre kilogrammes, so that the residue of 104,298 metre kilogrammes represents nearly three times as much potential energy as useful work. The same method of calculation being applied to a labourer, shows that the 3.5 oz. of flesh-formers, applied to external dynamical work, would, after deduction, yield

In this estimation the carbon in urea is supposed to be oxidized into carbonic oxide; but it would be still more in favour of the view if urea were taken as the residue, and six atoms more of hydrogen were oxidized.