March 8, 1862.

PHYSICAL SCIENCE.

On the Molecular Dissymmetry of Natural Organic Products, by L. PASTEUR, Member of the Chemical Society of Paris.

(Continued from page 120.)

III. WHEN I began to devote myself to particular pursuits, I sought to fortify myself in the study of crystals, anticipating that I would derive assistance from it in my chemical researches. The most simple means, it seemed to me, was to take a somewhat extensive work upon crystalline forms, to repeat all the measurements, and to compare my determinations with those of the author. In 1841, M. de la Provostage, whose exactness is well known, published a handsome work on the crystalline forms of tartaric acid, paratartaric acid, and their saline combinations. I took hold of this memoir. I crystallised tartaric acid and its salts, and I studied the form, of their crystals. But in my progress, I perceived that a very interesting fact had escaped the learned physicist. All the tartrates I studied afforded decided indications of hemihedric faces. This particularity of form in the tartrates was not very evident. This may be readily conceived, since it had not yet been observed. But when, in one species, it appeared in doubtful characters, I always succeeded in rendering it more manifest, by recommencing the crystallisation and slightly modifying the conditions. Sometimes the crystals bore all the faces required by the law of symmetry, but hemihedrity is the frequent irregularities of crystals, which are never easily developed. Hence their results, deformities, arrests of development in this or that direction, faces accidentally suppressed, &c. Except under circumstances almost exceptional, the ascertaining of hemihedrity, especially in crystals of the laboratory, requires a very attentive study. Besides that, although hemihedrity may be possible in a form, although it may be a function of the internal structure of the body, it may not be externally mani est any more than is found upon every crystal of the cubic species all the forms compatible with the cube.

But be that as it may, I repeat, I found hemihedric

tartrates.

This observation would have been barren, probably without the following:

Let a, b, c, be the parameters of the crystalline form of any tartrate; a, B, Y, be the angles of the crystallographic axes. These are ordinarily right angles or a little oblique. Besides the relation of the two parameters, such as a and b, is nearly the same in the different tartrates, whatever may be their composition, the quantity of their water of crystallisation, the nature of their bases; alone differs sensibly. There is a kind of semi-isomorphism among all the tartrates. One might say that the tartaric group prevails, and impresses a stamp of resemblance between these different forms, in spite of the difference of the other constituent elements.

It follows from this that there is something in common in the forms of all the tartrates, and it is possible to arrange them similarly, in taking for example, as character of similar position, the position of the axes a and B.

Now, if the disposition of the hemihedric faces be compared on all the prisms of the primitive forms of the tartrates, arranged in the same manner, this disposition is found to be the same.

Let us sum up, in two words, these results, which

133

have been the point of departure in all my ulterior researches; the tartrates are hemihedric, and they are so in the same direction.

Guided, on the one hand, by the fact of the existence of molecular rotary polarization, di-covered by M. Biot, in tartaric acid and in all its combinations; on the other by the ingenious approximation of Herschel; in the third place, by the learned views of M. Delafosse, with whom hemihedrity has always been a law of structure and not an accident of crystallisation, I presumed there might be a correlation between the hemihedrity of the tartrates and their property of deviating the plane of polarized light.

It is important to seize here the sequence of ideas. Haüy and Weiss ascertained that in quartz there exist hemihedric faces, and that these faces fall to the right in certain specimens, and to the left in others. On his side, M. Biot found that crystals of quartz are also divided into two groups, under the relation of their optical properties, some deviating to the right, and the others deviating to the left, the plane of polarized light according to the same laws. Herschel comes in his turn, places between these two facts, until then isolated, a line of connexion, and says:-The plagihedrals of one direction deviate in the same direction; the plagihedrals of the other direction deviate in the opposite direction. For my part, I find that all the tartrates are plagihedral, if I may so express myself, and that they are all in the same direction. I should then presume that here, as in quartz, there was a correlation between hemihedrity and circular polarization. At all events, the essential differences which I have just noticed between the circular polarization of quartz and that of tartaric acid ought not to be neglected.

We are now, thanks to the new facts which precede, and to the approximations which I have just enumerated, in possession of a preconceived idea (for it is, as yet, nothing more) on the possible correlation of hemihedrity and the rotary power of the tartrates.

Very desirous to find in experiment a corroboration of this still speculative proof, my first thought was to ascertain whether the very numerous crystallisable organic products, which possess the molecular rotary property, have hemihedric crystalline forms, which no one suspected, notwithstanding the approximation of Herschel. This investigation had the success which I anticipated for it.

I occupied myself also with the examination of the crystalline forms of paratartaric acid and its salts, substances isomeric with tartaric combinations, but all of which M. Biot found inoperative upon polarized light. None was found hemihedric.

The idea of the correlation of hemihedrity, and the molecular rotary power of natural organic products, gained ground. I was soon led to develope it clearly by a very unexpected discovery.

(To be continued.)

Chemical Society.-Arrangements have been made for the delivery of the following Discourses during the present year: April 3, H. Débus, Ph.D., F.R.S., "On the Influence of the Quantitative Method in the Development F.R.S.E., "On the Chemistry of Opium." June 5, W. of Scientific Chemistry." May 1, T. Anderson, M.D., Marcet, M.D., F.R.S., " On the Chemical Phenomena of Digestion." November 6, A. W. Hofmann, LL.D, F.R.S., "On Vapour-Densities." A Fifth Lectu be delivered by Prof. Wurtz, of Paris

robably

PROCEEDINGS OF SOCIETIES.

ROYAL INSTITUTION OF GREAT BRITAIN.

A Lecture by Dr. W. ODLING, F.R.S., "On Mr. GRAHAM'S Researches in Dialysis," delivered at the Royal Institution on Friday, February 14, 1862.

BEFORE proceeding to unfold to you in regular order the subject which I have undertaken to bring under your notice this evening, I wish to direct your attention for a few moments to one or two experiments which, as some little time is required for their performance, I will now set in operation, so that they may be ready for me to refer to at a later part of my address. The instrument which I hold in my hand is called a Dialyser. It is constructed somewhat like a tambourine. It consists of a piece of membrane, or vellum, or parchment paper stretched tightly over a hoop of gutta-percha, so as to form a sort of tray capable of holding water, and capable of being floated upon water. In each of these jars is a dialyser of similar construction to the one in my hand, but somewhat smaller. We have in this bottle a quantity of the red colouring matter obtained from coal tar, known as Magenta; and I am about to pour a little of it into one of these dialysers [A small portion of the Magenta was floated in a dialyser.] Now, I have here another red colouring matter, though a much less brilliant one; nevertheless it is possessed of very considerable tinctorial power, as I can show you by pouring a small quantity of it into a glass of water. This is the colouring matter of blood. You perceive that a very little of it is sufficient to impart a considerable degree of colouration to a large body of liquid. Now, I will pour some of this other red colouring matter-the colouring matter of blood-into this second dialyser,-[referring to one floating in another jar.] I have here a brown colouring matter, caramel, obtained by the roasting, or rather, heating of sugar. Its colour, I suppose, will be tolerably apparent at a distance. I will mix some of this brown colouring matter with the Magenta, and pour the two into the dialyser floating on the water in this tall jar [referring to a third]. We will allow the whole to remain undisturbed some little time, and at a later period of the lecture return to our examination of the jars in order to see whether or not any effects have taken place.

And now, by way of introduction to the proper subject of my story this evening, I will recall to your recollection that intermixture, or diffusion, which takes place when two different liquids are in contact with one another. I have here a tube bent in the shape of the letter U, such a one as is ordinarily known in laboratories as a U-tube. Into one limb I have already introduced a weak solution of common salt, which, for the sake of greater distinctness I have coloured pale blue. Into the other limb I now pour a solution of another salt,-namely, Epsom salts, or sulphate of magnesia, of about equal strength, which, for the sake of distinctness, I have coloured red. We have now in one limb of the tube a weak solution of common salt, coloured blue, and in the other limb of the tube a weak solution of Epsom salts, coloured red. The question is, whether these two salts will remain separate, as they now are, or whether they will mix with one another. We find, as a matter of observation, that after a certain time a mixture does take place A slow diffusion of the one into the other occurs; the common salt passes into the sulphate of magnesia limb, and the sulphate of magnesia passes into the common salt limb, until a perfect uniformity of composition in the liquid is established. I may show you the experiment in a somewhat different manner. Here, instead of taking a double-limbed tube, I take a double-bulbed tube. The lower bulb is filled with a solution of common salt, coloured pale blue. I will now pour in some water; and here we shall have to consider the same question as before,

Will the heavy common salt remain at the bottom of the tube, or will it gradually rise up into the light water? Here, as in the other instance, repeated observation tells us that the heavy common salt will not remain at the bottom of the tube, as, by its superior gravity, we might expect it would, but that it will, in opposition to the law of gravity, so far as that alone is concerned, rise up through the light water. Now, this experiment, although a very well-known one, is, nevertheless, of considerable interest. We have to inquire why the common salt, which is heavy, should not remain at the bottom, instead of rising up to the top through the light water. We admit generally that, in order to set a body in motion, the action of some external force is required. Now, I would ask, what is the external force which causes the heavy common salt to rise up through the light water? I have poured in this water somewhat roughly, and thereby have created a greater disturbance of the liquids than I intended, so that some of the blue solution of common salt has, by a mere accidental shaking, got into the upper bulb; but, quite irrespective of that shaking, a quantity of the common salt would gradually rise into the upper bulb and into the stem above it.

This rising up of the common salt into the water is termed its diffusion. The phenomena of liquid diffusion were first examined some fifteen or sixteen years ago by Mr. Graham, who has, with more or less of interruption, continued his researches upon the subject down to the present time; and it is to some of his results that I propose to direct your attention this evening.

His first experiments were conducted by means of a process which he termed "vial diffusion." The substance was allowed to diffuse from a small vial or cylinder. The experiment was conducted somewhat in this manner :-I have here two glass jars, one within the other. The internal jar is filled nearly to the top with a solution of chloride of copper-the salt whose diffusiveness is to be made the subject of experiment. I may here direct your attention to a similar experiment which is going on in the actual kind of apparatus used by Mr. Graham. In the internal cylinder or vial of this jar we are causing the diffusion to take place, not of the green salt, chloride of copper, but of the yellow salt, bichromate of potash. You see the interior vial containing the bichromate of potash solution, and the exterior jar the water. Well, the experiment was condu ted in this manner. The interior cylinder was filled with the solution of the salt whose diffusiveness was to be ascertained; it was then introduced gradually into an external jar of water, and the whole was set aside for some time. During that time a portion of the salt contained in the interior vial diffused itself out into the exterior liquid. The experiment was terminated by withdrawing the interior vial gradually and then ascertaining, by evaporating down the external liquid, or by some other means, how much of the salt from the vial bad got into the external water. This was the mode in which his original experiments were conducted. I may say, with regard to these two experiments which are in progress in the jars to which I have just referred, that they were arranged this morning at about eleven o'clock, and you perceive that very little, if any, perceptible diffusion has taken place Nevertheless, some of the salt has in each instance passed into the external liquid, as we might very readily prove.

Now, among the very many interesting results to which this very simple process gave origin, there are only two or three which have a direct bearing upon the subject of my lecture this evening, and of them I will now speak.

The first general conclusion arrived at was thisthat different salts differ very much from one another in the rapidity with which they diffuse. That is shown in this table labelled "Vial Diffusion." A series of these vials, perfectly similar to one another, having the same capacity, and having, more particularly, the same area of

CHEMICAL NEWS,

March 8, 1862.

opening, were filled with the same quantities of solutions of the same strength of different substances; one with a 20 per cent. solution of common salt, one with a 20 per cent. solution of sulphate of magnesia, one with a 20 per cent. solution of nitrate of soda, and another with a 20 per cent. solution of gum. They were introduced respectively into jars of water of this size and character-possibly into this identical jar-and allowed to diffuse for eight days at the temperature of 60°. This table [pointing to the subjoined] gives some of the results obtained :Vial Diffusion.

Diffusate.

58.68
27°42
51°56
26.74

13'24
3.08

It was found that from the 20 per cent. solution of chloride of sodium or common salt, 58.68, or rather more than 58 grains, had passed out into the larger jar-had diffused in fact. Now, thie quantity which had passed out into the external jar was spoken of by Mr. Graham as the diffusate. He found that from the 20 per cent. solution of sulphate of magnesia not quite 27 grains had diffused; from the solution of sugar 26 74 grains had diffused; from the solution of gum 13 grains; and from that of the albumen only 3 grains.

Now, it is obvious from a mere inspection of this table that the process of diffusion might be made available to effect, at any rate, a partial separation of substances-a partial separation, for instance, of a highly diffusive substance, like chloride of sodium, from a feebly diffusive substance, like albumen. Thus, if we were to take a solution containing equal weights of albumen and common salt, and were to pour it into a jar of water, the ratio of the albumen to the common salt would be the same in the dilute liquid resulting from the mixture of the water into which we pour it and the solution, as it would be in the original solution itself. But if, instead of pouring the solution into a mass of water, we were to pour it into one of these vials, and then allow it to diffuse into a mass of water, a very different result would take place: for every 58 grains of common salt that passed out there would be only 3 grains of albumen pass out into the external vessel, so that whilst in the liquid into which we poured the solution we should have the two compounds in the ratio of 1 to 1, or 100 to 100, in the liquid into which we allowed it to diffuse the ratio of common salt to albumen would be as 58 to 3, or as 100 to about 5.

But even this is not all. Not only are we capable by diffusion of effecting a partial separation of bodies which are merely mixed together, but we are also capable of effecting the decomposition of definite chemical compounds. I have here a piece of alum, a remarkably definite and crystalline body. It is a double sulphate of the two bases potash and alumina. Now, if we make a solution of this alum, and pour it into one of these vials, and introduce the vial into a diffusion-jar, and allow diffusion to take place, the tendency of the potash to diffuse being much greater than that of the alumina, the potash actually breaks away from the alumina with which it was in com. bination, in order to diffuse itself into the external water; so that at the termination of the experiment we find in this exterior liquid a certain quantity of free sulphate of potash, whereas in the internal vial we find a certain quantity of free sulphate of alumina, which sulphate of potash and sulphate of alumina have resulted from the chemical decomposition, that is, the breaking up of the chemical compound, alum, effected by the superior tendency which the potash had to diffuse over the tendency which the alumina had. And so in a great number of other instances, actual decomposition may be produced by the process of diffusion.

135

We

Mr. Graham's later series of experiments on diffusion were conducted in a somewhat different manner. A certain quantity of water was introduced into a jar, and the saline solution,-the solution of salt, whose diffusiveness was to be ascertained,-was then carefully conveyed to the bottom of the jar by means of a pipette. I have here the experiment on a somewhat large scale. have constructed a pipette out of a separater, and we will now allow a quantity of common salt solution (which, for the sake of distinction, I have coloured red) to pass down through the water. We are now conveying it to the bottom of the jar, where, you perceive, it forms a distinct layer. I have made this experiment on a somewhat larger scale than Mr. Graham employed, in order that it may be seen over the theatre. Here, however, we have one of Mr. Graham's jars, in which an experiment was begun early this morning. It will be rather difficult for me to render it visible all over the theatre, but some of you will see that there is here at the bottom of the jar a distinct layer of yellow liquid, namely, a solution of bichromate of potash, which was introduced this morning in much the same manner as I am now introducing the red liquid. The salt so introduced is allowed to diffuse into the superincumbent water. The experiment is conducted for a certain number of hours or days, and at the end of that time the exterior liquid is very carefully drawn off from the exact top of the liquid by means of a syphon, in definite layers. For instance, in this mode of conducting the experiment, there were 700 cubic centimètres of the liquid, and 100 cubic centimètres of the solution of the salt to be diffused. After the diffusion had gone on for some days, the top 700 centimètres of liquid were drawn off in portions not of 100 but of 50 centimètres, so as to make 14 portions or layers of 50 cubic centimètres each; a double layer of 100 centimètres being left at the bottom. The amount of salt which had diffused into each layer was then ascertained by evaporating each one separately, or the amount of salt contained in each layer was determined in some other manner. Some of the results obtained in this way are given in the table. I should say that, for the sake of distinguishing between the two processes, Mr. Graham designated this one (that of the former experiment) as vial diffusion; while this (the experiment last described) he called jar diffusion. These, then are some of the results of jar diffusion :—

Let us consider the results obtained with a solution of common salt. Originally there were 10 grammes of common salt introduced into the 100 centimètres of liquid which occupied the lowest layers. In the top layer we now find only th of a gramme of salt; in the 5th layer we find nearly ths; in the 7th layer ths; in the 8th layer foths; in the 12th layer ths; in the 13th layer 18ths; and in the 14th layer 15ths of a gramme of salt. The two original layers, the 15th and 16th, were not separated, but examined together, and in them were found ths of a gramme of the salt. Now, if we compare

this diffusion of common salt with the diffusion of albumen which took place under precisely the same conditions, we find that, whereas there was a considerable quantity of common salt in the topmost stratum of the liquid into which that substance had diffused, the albumen in the same time did not rise higher than the eighth stratum; and whereas in this case we introduced ten grammes of common salt into the lowest hundred cubic centimètres of liquid, and found that out of those 10 grammes 7 had passed out; in the case of the albumen we likewise introduced 10 grammes into the lowest 100 cubic centimètres, and found that less than 3 grammes had diffused into the superior strata. Perhaps these results will be rendered more evident by means of this diagram, which represents sections of the two jars. In the case of the common salt the whole of the two lower layers were at first occupied by the solution of common salt: at the end of the experiment we find that the common salt has actually risen up to the very top of the liquid, and could have risen considerably higher, as is shown by the fact that the quantity of common salt contained in the top layer of liquid into which it diffused is 20 times as great as the quantity of sulphate of magnesia contained in the top layer of liquid into which it diffused under precisely the same conditions. The common salt then not only rose to the top, but it could have risen higher, whereas the albumen rose only to the top of the eighth stratum. And whereas in the one case we have little more than two grammes of the common salt left in the lowest two layers, in the other case we have more than 7 grammes of the albumen, showing the great difference in the diffusive nature of salt and albumen. Of all the bodies in this table salt is the most highly diffusive; then sugar and sulphate of magnesia, which are much alike; gum is still less diffusive, and tannin much the same as gum; albumen, again, is still less; and caramel still less than albumen. Now, these results of jar diffusion bear out generally those of vial diffusion. We are capable of obtaining the same separation of salts from one another; and although my time is getting on very rapidly, I must for one moment direct your attention to the actual separation that was in this way effected. This larger table of the jar diffusion of mixed salts refers to an experiment in which a 5 per cent. solution of common salt was mixed with a 5 per cent. solution of the analogous chloride of potassium; and the two salts were allowed to diffuse together for a period of 7 days at the temperature of 55 degrees. Now, it was known that chloride of potassium diffused more rapidly than chloride of sodium. The results obtained by vial diffusion had shown that they diffused in about the proportion of 10 to rather more than 8. Now, the uppermost 6 strata of the liquid, into which the diffusion took place, were found to contain, altogether, at the end of 7 days, 561 milligrammes of the mixed salts, and out of that 561 milligrammes, 404 were chlorice of potassium; so that these 6 upper layers contained, altogether, 72 per cent. of chloride of potassium. At the beginning of the experiment, the liquid contained of chloride of potassium and chloride of sodium in the proportion of 50 to 50; but in these top 6 layers we obtained a mixture of the two salts in the proportion of 72 per cent. of the one, and 28 per cent. of the other.

Now, this [pointing to another table] is a similar experiment, conducted with the same base, but with different acids. Sulphate of soda and chloride of sodium were mixed in solution and allowed to diffuse for 7 days at a temperature of 55, and with these results. I should say that the difference in diffusibility between the sulphate of soda and the chloride of sodium is very much greater than that between chloride of potassium and chloride of sodium, being in the proportion of 10 to 7 instead of 10 to 8. The first 6 strata contained altogether 263 milligrammes of mixed salts, and of those 263 milligrammes, 239 were common salt, and the remainder only was sulphate of soda or Glauber's salt; so that nearly 91 per cent. of the mixed

salt contained in the 6 topmost layers was chloride of sodium, and only the remaining 9 per cent. was sulphate of soda. (To be continued.)

Friday, January 17.

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

A Paper, by Professor TYNDALL, F.R.S., &c., was read "On the Absorption and Radiation of Heat by Gaseous Matter." Resuming, with a new apparatus, his experiments on the influence of Chemical Combination on the Absorption and Radiation of Heat by Gases, the speaker, in the investigation of which the evening's discourse would be a résumé, first examines the deportment of chlorine as compared with hydrochloric acid, and of bromine as compared with hydrobromic acid, and finds that the act of combination which in each of these two cases notably diminishes the density of the gas, and renders the coloured gas perfectly transparent to light, renders it more opaque for obscure heat. He also draws attention to the fact that sulphur, which is partially opaque to light, is transparent to 54 per cent. of the rays issuing from a source of 100 C., while its compound, heavy spar, which is sensibly transparent to light, is quite opaque to the rays from a source of 100 C. He demonstrates, in confirmation of Melloni, the transparency of lampblack in thin layers; but shows how irreconcilable its deportment to radiant heat is with the idea generally prevalent at the present day, that lampblack absorbs heat of all kinds with the same intensity. All his experiments with gases have been repeated with a different source of heat, and he finds the result still more pronounced than formerly, that the compound gases far transcend the elementary ones in absorptive power. Taking air as unity, ammonia, at thirty inches tension, is 1195. this latter figure representing all the heat that issued from the source. A layer of ammonia, three feet long, is perfectly black to heat emanating from an obscure source. The coloured gases, chlorine and bromine, though much superior in absorptive power to the transparent elementary gases, are exceeded in this respect by every compound gas that has been hitherto examined. When, instead of tensions of thirty inches, we compare tensions of one inch, the differences between the gases come out still more strikingly. At this tension, for example, the absorption of sulphurous acid is 8000 times that of air. The speaker also referred to a new and extensive series of experiments on the Absorption of Radiant Heat by Vapours. The least energetic, as before, he finds to be bisulphide of carbon; the most energetic, boracic ether. He shows that the absorption of the latter vapour (which is quite transparent) at o'r of an inch of tension is 600 times the absorption of the densely coloured vapour of bromine, while in all probability it is 186,000 times that of air. The speaker was led by a series of perplexing experiments, which are fully described in a memoir recently presented to the Royal Society, to the solution of the following remarkable and at first sight utterly paradoxical problem :—“To determine the absorption and radiation of a gas or vapour without any source of heat external to the gaseous body itself." When air enters a vacuum it is heated by the stoppage of its motion; when a vessel containing air is exhausted by an air-pump, chilling is produced by the application of a portion of the heat of the air to generate vis viva. call the heating in the first case dynamic heating, and the chilling in the second case dynamic chilling. Let us further call the radiation of a gas which has been heated dynamically, dynamic radiation, and the absorption of a gas which has been chilled dynamically, dynamic absorption. Placing a thermo-electric pile at the end of his experimental tube, the latter being exhausted, the gas to be examined is permitted to enter the tube; the gas is heated, and if it possess any sensible radiative power, the

Let us