Now if the spontaneous act of decomposition of the peroxide of hydrogen were, as such, the immediate cause of the decomposition of the oxide of gold, we should expect that the action would not be greater than the cause; that is to say, that the decomposition of the more permanent oxide of gold, or of the peroxide of hydrogen itself, would not be more active than the original spontaneous decomposition of the latter compound.

But since, as experiment shows, the noble metals effect the decomposition of peroxide of hydrogen with just as much activity as their oxides, it must be assumed that in these cases the cause of decomposition lies in the metals themselves. From their elementary nature, however, these bodies cannot be in a state of decomposition, and they cannot therefore transfer such a state of decomposition to the peroxide of hydrogen; and hence the immediate cause of the more rapid decomposition of these bodies effected by metallic gold, &c., must be sought for in something else than in the transference of the chemical activity of one substance to another. However this may be, it appears possible that the action exerted by gold and by its oxide upon peroxide of hydrogen arises from one and the same cause, and that HO2 is directly decomposed by oxide of gold.

I have previously shown, and have here more fully mentioned the fact, that a great number of substances which decompose peroxide of hydrogen into water and O possess also the property of converting free Ŏ into O; and the oxides of the noble metals, and the metallic peroxides, belong more especially to this category. For this reason I am of opinion that the immediate cause of the decomposition of the HO2 effected by these substances lies in the allotropic modification which they produce in the Ŏ of this compound.

But since not only HO+Ỏ loses the half of its oxygen, but the oxide of gold, the oxygen of which is O, is reduced also, I must assume that the transformation of the O of HO+O into O causes also the same change of the Ŏ of AuŎ5; that is, that both compounds containing Ŏ mutually decompose each other, without being able to adduce any reason for it.

Mitscherlich, if I mistake not, has succeeded, by observing certain precautions, in decomposing 300 parts of sugar into alcohol and carbonic acid by means of 1 part of yeast. And this distinguished chemist has drawn from thence the conclusion, that the fermentation of sugar has no necessary connexion with the destruction of the yeast. He therefore thinks that if we could remove the causes which produce the destruction of the yeast, which are independent of the saccharine fermentation as such, we might by a given quantity of yeast effect the change of any quantity of sugar into alcohol and carbonic acid, just as we

can change any quantity of peroxide of hydrogen into water and common oxygen by means of the smallest quantity of gold.

It appears to me as if the actual facts of the phænomena of fermentation were by no means so accurately known as ought to be the case, to enable us to decide with certainty whether the chemical state of decomposition of the yeast, as such, exercises a determinate influence on the fermentation of the sugar or not.

If, as I am inclined to think, the transference of the chemical activity of one substance to another in the above-mentioned cases is only apparent, that is, mediate, we may consider it possible that the relation of the decomposition of the sugar to that of the yeast is not one of effect to canse; the latter decomposition is only concurrent with the former, and is not a phænomenon inducing the saccharine fermentation.

In what, then, is the obvious action of the yeast to be sought? I do not conceal, that to me our present knowledge of the fundamental phænomena of chemistry appears so imperfect, that I must deny to it the capability of solving the enigma of even the most proximate cause of fermentation; and in my opinion it is of more advantage to science to consider a phænomenon as still unexplained, than to assign to it a false cause; because the former excites to further inquiry, the latter retards or even puts an end to it.

We are certainly justified in considering as possible causes those still unexplained facts which we meet with in the phænomena of fermentation; and making use of this liberty, I will seek finally to indicate a possible reason for the activity of ferments.

I have adduced, in this and in other papers, facts which appear to me to show that a series of chemical actions, both analytical and synthetical in their nature, are caused by the allotropizing influence which certain substances exert upon oxygen. It is possible that the efficacy of the yeast might be due to an allotropizing influence which it exerts on one or the other of the constituents of sugar; and I will endeavour to explain this idea by adducing the various decompositions of which an inorganic compound, iodate of potash, is capable.

This salt decomposes, when sufficiently heated, into iodide of potassium and common oxygen; intimately mixed with graphite or peroxide of manganese, it decomposes into free iodine, oxygen and potash, at a lower temperature than that at which the salt alone decomposes in the manner stated.

As I have already remarked, the immediate reason of the last mode of decomposition of the iodate lies, in my opinion, in the deozonizing influence which peroxide of manganese, &c. exerts upon the O of iodic acid at a temperature which is far below

that at which iodate of potash, when alone, decomposes into: iodide of potassium and oxygen, or that at which free iodine decomposes with potash into iodide of potassium and oxygen.

Now if the breaking up of iodate of potash into iodine, oxygen and potash, is really occasioned by a change of condition which the Ŏ of iodic acid suffers under the influence of peroxide of manganese, &c.; if there are many other cases of the decomposition of compounds containing Ŏ, and if it be further the fact that organic substances also, as such, can exert an allotropic action upon common oxygen by inducing a series of chemical changes in other organic substances, it might also appear probable that yeast acts allotropically upon one or the other constituent of sugar, or so as to produce a change of condition.

Assuming that such an influence were really exerted, sugar could just as little retain its original chemical condition as iodate of potash could, after its Ŏ had experienced the deozonizing influence of peroxide of manganese, &c. The original chemical relations of the constituents of sugar to each other would be changed, and a new mode of combination would be produced.

We know for certain that two of the elements of sugarcarbon and oxygen-are capable of allotropy; and with respect to hydrogen, it may also exist in various conditions, as is indeed assumed by some chemists. But if it has been shown that the elements which form essential constituents of organic substances are capable of allotropy in an isolated condition; if, further, it cannot be doubted that one and the same element can exist, even in combination, in various allotropic conditions; and if there are many reasons for the assumption that not only heat, light and electricity, but also ponderable agents, act allotropically in certain free as well as chemically combined elements, we cannot avoid the conclusion that a great number of chemical reactions depend upon allotropic modifications of the elements, and especially that that class of phænomena might take place in organic substances, from the fact that their essential constituents can assume allotropic conditions.

In what form the elements exist in chemical compounds is at present a complete secret to us, and the processes which take place in analysis and synthesis with respect to these changes of condition, are still entirely unknown.

It is indeed pretty generally assumed, that it is an essential part of the nature of a chemical element to remain entirely unchanged in its smallest particles; and therefore it is assumed that in all chemical combinations and decompositions the elements remain entirely unchanged, and that the chemical process is purely mechanical, and generally depends on the definite manner in which the atoms of various elements group themselves, which

atoms are supposed to be acted upon by heat, light, electricity, affinity, &c.; in short, by agents which are assumed to lie outside the atoms, and as it were adherent to them.

The discovery of allotropy has made us acquainted with a changeability of the atoms themselves which we had not imagined, and that these can undergo such complete alterations, that, for example, a substance in an allotropic condition easily combines with a certain body, while in another condition it would be perfectly indifferent towards that body.

An attempt has indeed been made to explain the allotropic conditions of substances by a mechanical hypothesis, that is, to assume the existence of "Arrangemens particuliers des molecules," without, however, stating in what way such an "arrangement particulier" exists, or how such an essential change of the whole properties of a body can be caused by it.

Such attempts at explanation are, in my opinion, of little advantage to science; and I consider it advisable rather to say nothing about the obscure cause of allotropy than to build up hypotheses, which themselves are founded upon an hypothesis.

However unintelligible the phænomenon of allotropy may at present appear to us, so much is evident, that it has a great significance for theoretical chemistry; and I cannot avoid again remarking, that, in my opinion, the next considerable step in this science will consist in making out the influence which the allotropic modifications of elementary substances exert upon their chemical deportment, and especially upon chemical decompositions and combinations. And I think, therefore, that our insight into the processes of fermentation, and into so many other chemico-physiological processes and changes of matter, will only emerge from its present imperfect condition, and take the shape of real knowledge, when the connexion indicated between allotropy and chemistry is better and more thoroughly investigated than at the present day.

LXI. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from p. 384.]

June 19, 1856.-The Lord Wrottesley, President, in the Chair. HE following communications were read:

"Description of an Instrument for registering Changes of Temperature." By Balfour Stewart, Esq.

It lately occurred to the author that advantage might be taken of the capillary action of mercury to construct an instrument similar to a thermometer, but in which the mercury should expand from heat only in one tube, and contract from cold only in another. Accord

ingly a bulb was blown between two thermometric tubes of differentlysized bores, in such a manner that the tubes lay in one straight line, with the bulb between them. The bulb was then filled with mercury, and the tubes were hermetically sealed at both ends, having been first carefully deprived of air. When the instrument thus constructed was laid in a horizontal position, or nearly so, its action was precisely what the author had hoped; the mercury contracting from cold only in the narrow bore, and expanding from heat only in the wide one,-even when viewed by a microscope of considerable magnifying power.

It was suggested by Mr. Welsh, Director of the Kew Observatory, that such an instrument might be used for measuring fluctuations of atmospheric temperature; and the following use afterwards occurred to the author. Were it required to exactly estimate the radiating effect of a source of heat, it might perhaps be done by placing this instrument near the source, alternately exposing it to the influence of the calorific rays proceeding from the source, and intercepting these by means of a screen. Owing to the peculiar action of the mercury alluded to, the effect of the rays would be multiplied by the number of times the screen was interposed, provided it were always retained long enough to permit the mercury to cool down. The comparison of an instrument thus acted upon with another similar instrument near it, screened entirely from the source of heat, might furnish us with a means of exactly estimating the heating effect of the source.

The author desires to express his obligation to Mr. Welsh, who, besides finding a use for the instrument, suggested the selection of tubes which appears to answer best, and whose experience was of great assistance in arranging details. He is also indebted to the Kew Committee of the British Association, who kindly examined the instrument, and authorized the construction of several by way of trial. Mr. Casella undertook the operative part in their construction, and his glass-blower, M. J. E. Griffin, took pains to discover some of the circumstances that interfere with the proper action of the instrument, and constructed those that have proved successful.

Without attempting to explain all the peculiarities of this action, it would seem that the mercury is kept in the narrow bore, and prevented from retreating into the bulb, by friction; but, when a moving force is supplied by means of a change of temperature, the motion of the mercury takes place in that direction in which it is least opposed, or most aided, by its capillary action.

As the result obtained is due to the difference between two forces, neither of which is very great, the construction of such an instrument requires care; and the author will now state what appear to be the chief points which demand attention, as derived from his own experience, and that of those who have interested themselves in the construction of the tubes; although this experience is necessarily very limited.

1st. The tubes should be quite clean and free from moisture.

2nd. They should be in one straight line, and should expand symmetrically into the bulb.