August 24, 1882] 



NA TURE 



405 



tinually diminishing, and must in lime become too slow to 

 produce any sensible projection of the atmo-phere into distant 

 regions of planetary space. As to the chemical - iJe of the theory, 

 Dr. Siemens supposes the gases whic 1 pervade the plane'.ary 

 space to be not only of the same kind as the components of our 

 own atmosphere, whicb, on the kinetic theory of gases diffuse 

 through that space, but also such gases as are not found in 

 our air, but are found occluded in meteorites which may be 

 supposed to have acquired them in their previous wanderings. 

 Amongst these he specially mentions hydrocarbons which form 

 the self-luminous part of most comets. It is to these gases, 

 together with aqueous vapour, and carbonic acid, that he ascribes 

 the principal part in the conservation of solar energy. That 

 compound gases at the extremely low pressure of the planetary 

 space are decomposed by solar radiation is not inconsistent with 

 the laws of dissociation, for it is quite possible that some com- 

 pounds may be decomposed at ordinary temperatures by mere 

 reduction of pressure, and the radiation absorbed will be the 

 more effective, because it will directly affect the vibratory motion 

 within the molecule, and may well produce chemical decomposi- 

 tion before it can, when the free path of the molecules is so much 

 increased by the attenuation of the gas, assume the form of an 

 increased temperature. Dr. Siemens, moreover, adduces a 

 remarkable experiment in confirmation of his supposition. We 

 know, too, the power which our atmosphere, and especially the 

 water vapour in it, has of absorbing the infra-re.l rays, and that 

 amongst the Fraunhofer lines some of the strongest groups are 

 due to aqueous vapour, and the capital observation made by the 

 spectroscopic observers at the last total eclipse, that the group 

 of lines known as "B," which is one of those produced by 

 aqueous vapour, is greatly strengthened when the sun's light 

 passes by the edge of the mom ani sj through the lunar atmo- 

 sphere, may be taken as a confirmation of the theory that gases, 

 like our atmosphere, are diffused through space and concentrated 

 about the planets. But if it be true that the compounds are 

 decomposed by absorbing the sun's rays, we ou^ht to find in our 

 atmosphere the products of decomposition, we ought to find in 

 it free hydrogen, carbonic oxide, and acetylene or some other 

 hydrocarbons. The hydrogen from its small specific gravity 

 would not be concentrated in the lower regions of our atmo- 

 sphere in the same proportion as (he denser gases, but carbonic 

 oxide and hydrocarbons could not fail to be detected in the air if 

 they formed any sensible proportion of the gases in the planetary 

 space. That a large portion of the solar radiation is intercepted 

 before it reaches the earth, is no doubt true, for there are not 

 only the dark bands which are increased by our atmosphere, and 

 may reasonably be attributed in part to the action of like gases 

 pervading the space between us and the sun, but there is a con- 

 tinuous aborption of the ultra-violet spectrum beyond the line 

 U, and Cornu has found that this absorption is not sensibly 

 affected by out- atmosphere, so that the substance, whatever it 

 may be which produces it, may be an agent in the process 

 imagined by Dr. Siemens, but cannot be the means ot restoring 

 to the sun any portion of the radiant energy which reaches our 

 distance from him. 



Dr. Siemens expl ins the self-luminous character of comets by 

 the theory that the streams of meteoric stunes, of which they 

 are supposed to consi-t, bring fro n stellar space hydrocarbon 

 and other gases occluded within them ; and that in consequence 

 of the rise of temperature due t > the frictional resistance of such 

 a divided mass moving with enormous velocity, aided by 

 attractive condensation, the occluded gases will be driven out 

 and burnt, the flame giving rise to the original light emitted by 

 the nucleus. Now the spectrum of most comets shows only the 

 principal bands of a Bunsen burner, and is therefore adequ itely 

 explained by the flame of gas containing hydrocarbons, such as 

 have been found in meteorites. But Dr. Huggins has observed 

 in the spectrum of more than one comet not only hydrocarbon, 

 but cyanogen bands, and, although carbon and nitrogen com- 

 bine readily in the electric arc, a coal gas flame in air shows no 

 trace of the spectrum of cyanogen, and it would certainly put 

 some strain on our credulity if it were asserted that cyanogen 

 were one of the gases brought ready-formed by meteorites from 

 stellar space. Prof. Dewar and I have, however, recently shown 

 that if nitrogen already in combination, as, for instance, in am- 

 monia, be brought into a hydrocarbon flame, cyanogen is pro- 

 duced in sufficient amount to give in a pnotograph (but not 

 so as to be directly visible) the characteristic spectrum of cyano- 

 gen as it appears in the comets. It is therefore no longer neces- 

 sary to make any other supposition to account for the cyanogen 



bands in the spectra of comets than that ammonia or some such 

 compound of nitrogen is present, as well as hydrocarbons in a 

 state of ignition. 



Quite recently Dr. Huggins has observed that the principal 

 comet of this year has a spectrum of an entirely different cha- 

 racter, but he is not yet able to say to what elements or com- 

 pounds it i, probably due. The notion that comets may bring 

 us news of dista.it parts of stellar space, towards which our 

 system is driving, where the atmosphere is not like ours, oxygen 

 and nitrogen, but hydrogen and hydrocarbons, may fascinate the 

 fancy, but the laws of occlusion oblige us to think that the 

 meteorites have not merely wandered through an attenuated 

 atmosphere of hydrogen an I hydrocarbons, but have cooled in a 

 much denser atmosphere of these substances, which we can only 

 conceive as concentrated by the presence of a star or some laige 

 aggregation of matter. They may perchance have come from 

 some nebulous mass, for Draper and Huggins tell us that in the 

 great nebula in Orion, hydrogen is dense enough and hot enough 

 10 show some of its characteristic lines, besides the F line, which 

 is seen in other nebulae, and is the last to disappear by reduction 

 of density. No comet on visiting our system a second time can 

 repeat the exclusion of its occluded gas^s unless its store has 

 been replenished in the interval, and it will be interesting to 

 see when Halley's comet next returns, whether it shines only by 

 reflected light, or gives us, like so many others, the banded 

 spectrum of hydrocarbons. 



SECTION D 



BIOLOGY 



Opening Address by Arthur Gamgee, M.D., F.R.S.,. 

 Brackenbury Professor of Physiology in Owens 

 College, Manchester, President of the Section. 



On the Growth of our Knowledge of the Function of Secretion, 

 to which is prefixed a Brief Sketch of the Writings of the late 

 Professor Francis Mutt land Balfour. — When the Council of the 

 British A-sociation did me the honour of asking me to preside 

 over this section, it occurred to me that a suitable subject lor the 

 presidential address would be a Survey of the Growth of or.r 

 know ledge of the Function of Secretion ; for no subject, which 

 has recently been the object of minute study by animal physi- 

 ologists, is more likely to interest all devoted to biological 

 pursuits, however diverse. I accordingly propose to direct your 

 attention, for the greater part of the time at our disposal to-day. 

 to what appears to me to be the most important and the most 

 interesting of the researches bearing on this subject. 



Before, however, entering upon the proper subject of this 

 address, it would ill become me as president of this section were 

 I not to speak to you, however imperfectly, of two great losses 

 which we have sustained, and which have saddened, and still 

 sadden, the hearts of many of us. The year 18S2 will long be 

 memorable, and sadly memorable, as a year during which 

 English biology sustained irreparable losses. So much has lately- 

 been written concerning that veteran in science, Charles Darwin, 

 who will figure in the history of the human intellect with such 

 men as Socrates and Newton, that I feel no words of mine are 

 needed to add to your sentiments of admiration and respect. He- 

 has made for himself an imperishable reputation as one of the 

 subtlest, most patient, and most truthful observers of natural 

 phenomena. His powers as an ob-erver were, however, almost 

 surpassed by his ingenuity as a reasoner, and his power to frame 

 the hyputhe-es most apt, in the actual state of science, to re- 

 concile all the facts which came within the range of his observa- 

 tion. We remember the time when the name of Charles Darwin, 

 and the mention of the theories connected with his name, 

 awakened, on the part of many, sentiments of antagonism and 

 of unreasonable opposition. But we have lived to witness, what 

 I may term, a great reparation. Even those who did not know 

 the man, and the qualities of mind and heart which endeared 

 him to so many, have come to recognise that in his work he was 

 actuated by a single-hearted desire to discover the truth ; and, 

 after calm reflection, they have conceded that his studies and 

 his views like all studies and all views which are based upon 

 the truth, not only are not irreconcilable with, but add 

 to our conceptions of, the dignity and glory of God. 

 And here I may be allowed to remark that it is impossible 

 to study the writings of Darwin, and especially the one 

 in <\hich he treats of "The Descent of Man," without 

 recognising an undercurrent of reverent sentimtnt, which in one 



