1S53] MEETING OF THE BRITISH ASSOCIATION FOE THE ADVANCEMENT OF SCIENCE. 



69 



least disturbing it. From what is already known, we can predicate 

 that a ball of iron entering the atmosphere with a velocity af six or 

 seven miles a second would instantly be melted, burnt, und converted 

 into a red powder, and that before reaching the earth it would pro- 

 bably be scattered by the aerial currents into comparatively so vast an 

 area as never to be afterwards noticed. If we suppose the mechanical 

 force produced by the condensation of the nebulous mass from which 

 a planet is forming to be slower than the equivalent of radiation from 

 the same, it would seem as if there could be no great internal heat; 

 but it is to be remembered that the vertical law of conduction requires 

 an increase of temperature downwards, so that it a planetary mass 

 were exposed perfectly cold to the sun's rays, it must continue to 

 absorb heat until that vertical equilibrium of temperature had been 

 attained : — the centripetal energy enabling it to imbibe a quantity of 

 heat vastly greater than the surface temperature would seem to indicate. 

 In respect to extra-terrestial bodies such subterranean heat is latent. 

 With regard to the sun, on the other hand, the mechanical force gene- 

 rated centripetally must originally have far exceeded the equivalent 

 of radiation. If its present condition is stationary iu respect to tem- 

 perature, its mass must be increasing. If its mass is not increasing, its 

 temperature must be diminishing, the annual loss beiug represented 

 by about 1-54 millionth of its mass lowered 1,81)0 million of degrees. 

 per annum, supposing it to have the specified heat of iron : supposing, 

 also that it does not contract or become further condensed, because this 

 would of itself engender vis viva. It may be shown that so small an 

 increase of density as would diminish the sun's diameter 8G0 feet re- 

 presents the equivalent of the annual radiation. In the bodies that 

 surround us, we remark that cooling and contraction are generally 

 simultaneous. If such is the case in the sun, 33 degrees must be too 

 'high an estimate of the yearly loss of temperature. The ratio between 

 the diminution of bulk and of temperature, were it known in the case 

 of the sun, would enable us to compare their mechanical equivalents. 

 The vis viva produced by the diminution of bulk would be classed with 

 the phenomena of what is called latent heat in liquids, solids and gases. 

 It would seem from these computations, which rest upon M. Pouillet's 

 data, that the probable annual loss of temperature in the sun is by no 

 means inconsiderable in absolute amouut, but its relative value in res- 

 pect to the temperature of the sun may be,* and probably is quite 

 insignificant. Is there any way of arriving at an estimate of the tem- 

 perature of the sun's radiating surface ?. Let us consider what mean- 

 ing is to be given to the expression " temperature of space," occasion- 

 ally to be met with in the writings of physicists. If heat is the motion 

 of the elemeutarv parts of bodies, and not a subtle species of matter, 

 as certain phenomena of lateut heat seem to have suggested the idea, 

 it is hardly correct to speak of vacant space as having a temperature, 

 although the heat force may in various directions and with various 

 intensities be radiating through it. In the same way. space is not 

 considered as luminous, although traversed by most intense light. A 

 thermometer placed iu a perfect vacuum although it shows the same 

 temperatnre as the substance that incloses the vacuum, actually exhi- 

 bits the effect of the intensity of the heat radiations that are passing 

 through it. If we suppose a thermometer situated at the opposite point 

 of the earth's orbit, and subject to the influence of the sun's rays only, 

 it would no doubt rise until the radiation from its surface amounted to 

 -what was radiated into its surface ; but the temperature indicated by 

 it cannot be accepted either as. constant, for it depends on the specific 

 radiating and absorbing qualities of the thermometer ; or as affording 

 the means of deducing the sun's temperature, for we are ignorant of 

 the relation between temperature and the rate of emission, also of the 

 absolute value of auy given temperature unless we deduce it from the 

 dynamic theory of gases which represents the zero of gaseous tension 

 ( — 461° Fahr.) as the absolute zero of heat. If the thermometer thus 

 isolated, is supposed to be surrounded, on all sides but the one exposed 

 to the siin, by matter that is kept artificially heated up, to within a 

 few degrees of the temperature shown by the thermometer, it is im- 

 possible that it could receive an ticcession of heat from any other source 

 but the sun; and it seems obvious that when at last it became station- 

 ary, the temperature is one that must be independent of any specific 

 quality of the thermometer or its artificially heated envelope, but de- 

 pendent entirely en the distance and temperature of the sun. Some 

 years ago I made an attempt to imitate the conditions of this hypo- 

 thetical experiment by inclosing a thermometer within three concentric 

 boxes well protected from external influences, and capable of being 

 equally heated all round to any temperature below 4U'j° Fahr. by 

 means of flues ascending from an Argand lamp. The rays of the snn 

 when near the meridian, (within the Tropi-s) were admitted to fall 

 when required on the bulb of a thermometer through a triple glass par- 

 tition. Before applying the lamp, the temperature of the interior of 

 the box being t, a rise of about 50° took place by exposing the bulb to 

 the sun ; when the thermometer had become stationary at t -\- 50° the 

 sun's rays were excluded and the lamp applied to heat the box to t -4- 



50°. AVIien the temperature was again stationary at this point, the sun 

 was re-admitted upon the thermometer, which again rose 50° or until 

 the temperature was t -\- 100°. 'i he same operations were repeated 

 up to 250°, but without any diminution of the step 50° which seemed 

 to be made with the s.. me alacrity at the higher as at the lower tempe- 

 rature. I had hoped to have detected some very obvious difference, 

 and from its amouut to infer the value of the limiting temperature that 

 expressed the sun's power at the earth's distance. 1 should then have 

 added 4(i° to this temperature tc obtain its absolute value, then increase 

 this iu the inverse ratio of the square of the distau e from the sun's 

 centre, obtain an approximate value of the sun's temperature. It 

 seemed to me at the time that this experiment, though not made with 

 sufficient means, or perhaps, care to insure much accuracy, proved that 

 the intrinsic force of the sun's rays ot heat was mucli greater than 

 might be inferred from the temperature of the atmosphere. I purpose 

 at a future opportunity to consider the Dynamical Sequence of Latent 

 Heat and Molecular Force. 



Mr. Hopkius addressed the Section, pointing out the important hints 

 and valuable lines of inquiry which the paper suggested ; but also 

 showing with what caution it was to be be received in many parts as 

 statements of determined scientific truth. 



Section- B.— CHEMICAL SCIENCE. 



' On the Chemical Action of the Solar Radiations,' by Mr. R. Hunt 

 — This was a report to the section of the continuation of an examina- 

 tion of the chemical action of the prismatic spectrum, after it had been 

 subjected to the absorptive infinences of different coloured media. The 

 mode of examination has been to obtain well defined spectra of abeam 

 of light passing through a fiue vertical slit in a steel plate by prisms 

 of flint and crown glass and of quartz. The spectrum, being concentia- 

 ted by a lens, was received upon a white tablet and submitted to care- 

 ful admeasurement; the coloured screen (sometimes colomed glass 

 aud sometimes coloured fluid) was then interposed, and the alterations 

 in the chromatic image were carefully noted ; the chemical preparation 

 was ihen placed upon the tablet, and the chemical impression obtained. 

 The relation which this image bore to the luminous image was a true 

 representation of the connexion between the colour of a ray, and its 

 power to produce chemical cl ange. In the report made to the Belfast 

 meeting of the British Association, the results of experiments made 

 upon glass plates prepared by the so-called collodion process were 

 alone given. In the present report the examination has been extended 

 to the photographic preparation known as the calotype, and to iodide 

 and bromide of silver in their pure state and when excited by gallic 

 acid. M. Edmoud Becquerel, in a paper eommin.kated to the Acade- 

 my of Sciences, of which an abstract appears iu the Comptcs Jlcndus, 

 torn xvii. p. 883, states " that when any part of the luminous spectrum 

 is absorbed or destroyed by any substance whatever, the part of the 

 chemical rays of the same refrangibility is also destroyed." The au- 

 thor's experiments, as recorded in the former report and those now 

 detailed, prove that his conclusion has been formed too hastily. 

 Although there are many absorptive media which, at the same time 

 as they obliterate a particular coloured .ray, destroy the chemical 

 action of that portion of the spectrum, yet there are still more exten- 

 sive series which prevent the passage of a ray of given refi>angibility, 

 and do not, at the same time, obstruct those rays which are chemically 

 active of the same degree of refrangibility. This is particularly exem- 

 plified in the case of glass turned yellow by different preparations. 

 With some of these the blue rays are obliterated, the chemical action 

 of this part of the spectrum not being interrupted, whereas in some 

 other examples, those rays permeate the glass, but are almost entirely 

 deprived of chemical power. A still more curious fact is noticed in 

 this report, for the first time, of some media which have the power, as 

 it were, of developing chemical action in a particular part of the spec- 

 trum where the rays did not appear previously to possess this power. 

 Several glasses exhibited this phenomenon to a certain extent, particu- 

 larly such as were stained yellow by the oxide of silver; but one glass 

 showed this in a remarkable manner. This glass was yellow when 

 viewed by transmitted light, but it reflected pale blue light from one 

 cf its surfaces; it obliterated the more refrangible rays down to the 

 green, and rendered the yellow rays far less luminous than usual. In 

 nearly every case the yellow rays are found to be not merely inactive 

 chemically, but to actively prevent chemical action. After the spec- 

 trum has been submitted to the action of this glass, all chemical power 

 is confined to this yellow ray. The author has hitherto supported the 

 view that photographic phenomena and the illuminating power of the 

 sunbeam were distinct principles, united only in their modes of mo- 

 tion. He was led to this from observing that where there was the 

 m ost light there was the least power of producing chemical change ; 

 and that as illuminating power diminished, the chemical phenomena 



