39^ 



NATURE 



l[Sept. 7, 1876 



In the solar beam we have an agent of surpassing power, the 

 investigation of whose properties by Newton forms an epoch in 

 the history of experimental science scarcely less important than 

 the discovery of the law of gravitation in the history of physical 

 astronomy. Three actions characterise the solar beam, or, 

 indeed, more or less that of any luminous body — the heating, the 

 physiological, and the chemical. In the ordinary solar beam we 

 can modify the relative amount of these actions by passing it 

 through different media, and we can thus have luminous rays 

 with little heating or little chemical action. In the case of the 

 moon's rays it required the highest skill on the part of Lord 

 Rosse, even with all the resources of the observatory of 

 ParsonstowD, to investigate their heating properties, and to 

 show that the surface of our satellite facing the earth passes, 

 during every lunation, through a greater range of tempera- 

 ture than the difference between the freezing- and boiling-points 

 of water. 



But if, instead of taking an ordinary ray of light, we analyse 

 it as Newton did by the prism, and isolate a very fine line of the 

 spectrum (theoretically a line of infinite tenuity), that is to say, 

 if we take a ray of definite refrangibility, it will be found impos- 

 sible, by screens or otherwise, to alter its properties. It was his 

 clear perception of the truth of this principle that led Stokes 

 to his great discovery of the cause of epipolic dispersion, 

 in which he showed that many bodies had the power of absorb- 

 ing dark rays of high refrangibility and of emitting them as 

 luminous rays of lower refrangibility — of absorbing, in short, 

 darkness and of emitting it as light. It is not, indeed, an easy 

 matter in all cases to say whether a given effect is due to the 

 action of heat or light ; and the question which of these forces 

 is the efficient agent in causing the motion of the tiny discs in 

 Crookes's radiometer has given rise to a good deal of discussion. 

 The answer to this question involves the same principles as those 

 by which the image traced on the daguerreotype plate, or the 

 decomposition of carbonic acid by the leaves of plants, is referred 

 to the action of light and not of heat ; and applying these prin- 

 ciples to the experiments made with the radiometer, the weight 

 of evidence appears to be in favour of the view that the repulsion 

 of the blackened surfaces of the discs is due to a thermal reaction 

 occurring in a highly rarefied medium. I have myself had the 

 pleasure of witnessing many of Mr. Crookes's experiments, and I 

 cannot sufficiently express my admiration of the care and skill 

 with which he has pursued this investigation. The remarkable 

 repulsions he has observed in the most perfect vacua hitherto 

 attained are interesting, not only as having led to the construc- 

 tion of a beautiful instrument, but as being likely, when the 

 subject is fully investigated, to give valuable data for the theory 

 of molecular actions. 



A singular property of light, discovered a short time ago by 

 Mr. Willoughby Smith, is its power of diminishing the electrical 

 resistance of the element selenium. This property has been 

 ascertained to belong chiefly to the luminous rays on the red 

 side of the spectrum, being nearly absent in the violet ©r more 

 refrangible rays and also in heat-rays of low refrangibility. The 

 recent experiments of Prof. "W. G. Adams have fully established 

 the accuracy of the remarkable observation, first made by Lord 

 Rosse, that the action appeared to vary inversely as the simple 

 distance of the illuminating source. 



Switzerland sent, some years ago, as its representative to this 

 country, the celebrated De la Rive, whose scientific life formed 

 lately the subject of an eloquent eloge from the pen of M. Dumas. 

 On this occasion we have to welcome, in General Menabrea, a 

 distinguished representative both of the kingdom of Italy and of 

 Italian science. His great work on the determination of the 

 pressures and tensions in an elastic system is of too abstruse a 

 character to be discussed in this address ; but the principle it 

 contains may be briefly stated in the following words : — " When 

 any elastic system places itself in equilibrium under the action of 

 external forces, the work developed by the internal forces is a 

 minimum." General Menabrea has, however, other and special 

 claims upon us here, as the friend to whom Babbage entrusted 

 the task of making known to the world the principles of his 

 analytical machine— a gigantic conception— the effort to realise 

 which it is known was one of the chief objects of Babbage's 

 later life. The latest development of this conception is to be 

 found in the mechanical integrator of Prof. J. Thomson, in which 

 motion is transmit ed, according to a new kinematic principle, 

 from a disk or cone to a cylinder through the intervention of a 

 loose ball, and in Sir W. Thomson's machine for the mechanical 

 integration of differential equations of the second order. In the 

 exquisite tidal machine of the latter we have an instrument by 



means of which the height of the tide at a given port can be 

 accurately predicted for all times of the day and night. 



The attraction-meter of Siemens is an instrument of great 

 delicacy for measuring horizontal attractions, which it is proposed 

 to use for recording the attractive influences of the sun and moon, 

 upon which the tides depend. The bathometer of the same able 

 physicist is another remarkable instrument, in which the constant 

 force of a spring is opposed to the variable pressure of a column 

 of mercury. By an easy observation of the bathometer on ship- 

 board, the depth of the sea may be approximately ascertained 

 without the use of a sounding-line. 



The Loan Exhibition of Apparatus at Kensington has been a 

 complete success, and cannot fail to be useful both in extending 

 a knowledge of scientific subjects and in promoting scientific 

 research throughout the country. Unique in character, but most 

 interesting and instructive, this exhibition will, it is to be hoped, 

 be the precursor of a permanent museum of scientific objects, 

 which, like the present exhibition, shall be a record of old as 

 well as a representation of new inventions. 



It is often diiticult to draw a distinct line of separation between 

 the physical and chemical sciences ; and it is perhaps doubtful 

 whether the division is not really an artificial one. The chemist 

 cannot, indeed, make any large advance without having to deal 

 with physical principles ; and it is to Boyle, Dalton, Gay-Lussac, 

 and Graham that we owe the discovery of the mechanical laws 

 which govern the propenies of gases and vapours. Some of 

 these laws have of late been made the subject of searching 

 inquiry, which has fully confirmed their accuracy, when the 

 body under examination approaches to what has not inaptly 

 been designated the ideal gaseous state. But when gases are 

 examined under varied conditions of pressure and temperature, 

 it is found that these laws are only particular cases of more 

 general laws, and that the laws of the gaseous state, as it exists 

 in nature, although they may be enunciated in a precise and 

 definite form, are very different from the simple expressions 

 which apply to the ideal condition. The new laws become 

 in the turn inapplicable when from the gaseous state proper 

 we pass to those intermediate conditions which, it has been 

 shown, link with unbroken continuity the gaseous and liquid 

 states. As we approach the liquid state, or even when we reach 

 it, the problem becomes more complicated ; but its solution even 

 in these cases will, it may confidently be expected, yield to the 

 powerful means of investigation we now possess. 



Among the more important researches made of late in physical 

 chemistry, I may mention those of F, Weber on the specific heat 

 of carbon and the allied elements, of Berthelot on thermo-che- 

 mistry, of Bunsen on spectrum analysis, of Wiillner on the band- 

 and line-spectra of the gases, and of Guthrie on the chryo- 

 hydrates, 



Cosmical chemistry is a science of yesterday ; and yet it 

 already abounds in facts of the highest interest. Hydrogen, 

 which, if the absolute zero of the physicist does not bar the way, 

 we may hope yet to see in the metallic form, appears to be every- 

 where present in the universe. It exists in enormous quantity in 

 the solar atmosphere, and it has been discovered in the atmo- 

 spheres of the fixed stars. It is present, and is the only known 

 element of whose presence we are certain, in those vast sheets of 

 ignited gas of which the nebulae proper are composed. Nitrogen 

 is also widely diffused among the stellar bodies, and carbon has 

 been discovered in more than one of the comets. On the other 

 hand, a prominent line in the spectrum of the Aurora Borealis 

 has not been identified with that of any known element ; and 

 the question may be asked : — Does a new element, in a highly 

 rarefied state, exist in the upper regions of our atmosphere ? or 

 are we, with Angstrom, to attribute this line to a fluorescent or 

 phosphorescent light produced by the electrical discharge to 

 which the aurora is due ? This question awaits further obser- 

 vations before it can be definitely settled, as does also that of the 

 source of the remarkable green line which is everywhere con- 

 spicuous in the solar corona. 



I must here pause for a moment to pay a passing tribute to the 

 memory of Angstrom, whose great work on the solar spectrum 

 will always remain as one of the finest monuments of the science 

 of our period. The influence, indeed, which the labours of 

 Angstrom and of Kirchhoff have exerted on the most interesting 

 portion of later physics can scarcely be exaggerated ; and it may 

 be triily said that there are few men whose loss will be longer 

 felt or more deeply deplored than that of the illustiious astro- 

 nomer of Upsala, 



I cannot pursue this subject further, nor refer to the other 

 terrestrial elements which are present in the solar and stellar 



