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and all。 They were very attentive； and I never felt more comfortable in a lecture。 I am happy to say that； having administered a dose of cement to Mrs。 Nasmyth's friend； Sir Fireside Brick of Green Lanes； he is now in a convalescent state。 The lecture is to be repeated in another fortnight。 With many thanks for your kind assistance； yours very sincerely；
〃EDWARD COWPER。〃
In the course of my astronomical inquiries I had occasion to consider the causes of the sun's light。 I observed the remarkable phenomena of the variable and some times transitory brightness of the stars。 In connection with geology； there was the evidence of an arctic or glacial climate in regions where such cannot now naturally exist； thus giving evidence of the existence of a condition of climate； for the explanation of which we look in vain for any at present known cause。 I wrote a paper on the subject； which I sent to the Astronomical Society。 It was read in May 1851。 In that paper I wrote as follows：
〃A course of observations on the solar spots； and on the remarkable features which from time to time appear on the sun's surface； which I have examined with considerable assiduity for several years； had in the first place led me to entertain the following conclusion： namely； that whatever be the nature of solar light； its main source appears to result from an action induced on the exterior surface of solar sphere； a conclusion in which I doubt not all who have attentively pursued observations on the structure of the sun's surface will agree。
〃Impressed with the correctness of this conclusion； I was led to consider whether we might not reasonably consider the true source of the latent element of light to reside； not in the solar orb； but in space itself； and that the grand function and duty of the sun was to act as an agent for bringing forth into vivid existence its due portion of the illuminating or luciferous element； which element I suppose to be diffused throughout the boundless regions of space； and which in that case must be exhaustless。
Assuming； therefore； that the sun's light is the result of some peculiar action by which it brings forth into visible existence the element of light； which I conceive to be latent in； and diffused throughout space； we have but to imagine the existence of a very probable condition； namely； the unequal diffusion of this light…yielding element； to catch a glimpse of a reason why our sun may； in common with his solar brotherhood； in some portions of his vast stellar orbit； have passed； and may yet have to pass； through regions of space； in which the light…yielding element may either abound or be deficient； and so cause him to beam forth with increased splendour； or fade in brilliancy； just in proportion to the richness or poverty of this supposed light…yielding element as may occur in those regions of space through which our sun； in common with every stellar orb； has passed； is now passing； or is destined to pass； in following up their mighty orbits。
〃Once admit that this light…yielding element resides in space； and that it is not equally diffused； we may then catch a glimpse of the cause of the variable and transitory brightness of stars；and more especially of those which have been known to beam forth with such extraordinary splendour； and have again so mysteriously faded away； many instances of which abound in historical record。
〃Finally； in reference to such a state of change having come over our sun； as indicated by the existence of a glacial period； as is now placed beyond doubt by geological research； it appears to me no very wild stretch of analogy to suppose that in such former periods of the earth's history our sun may have passed through portions of his stellar orbit in which the light…yielding element was deficient； and in which case his brilliancy would have suffered the while； and an arctic climate in consequence spread from the poles towards the equator； and thus leave the record of such a condition in glacial handwriting on the everlasting walls of our mountain ravines； of which there is such abundant and unquestionable evidence。 As before said； it is the existence of such facts as we have in stars of transitory brightness； and the above named evidence of an arctic climate existing in what are now genial climates； that renders some adequate cause to be looked for。 I have accordingly hazarded the preceding remarks as suggestive of a cause； in the hope that the subject may receive that attention which its deep interest entitles it to obtain。
〃This view of the source of light； as respects the existence of the luciferous element throughout space； accords with the Mosaic account of creation； in so far as that light is described as having been created in the first instance before the sun was called forth。〃 Dr Siemens read a paper before the Royal Society in March 1882； on 〃A New Theory of the Sun〃。 His views in many respects coincided with mine。* ＇footnote。。。 Interstellar space； according to Dr。 Siemens； is filled with attenuated matter； consisting of highly rarefied gaseous bodies including hydrogen； oxygen； nitrogen； carbon； and aqueous vapour； that these gaseous compounds are capable of being dissociated by radiant solar energy while in a state of extreme attenuation； and that the vapours so dissociated are drawn towards the sun in consequence of solar rotation； are flashed into flame in the photosphere； and rendered back into space in the condition of products of combustion。 With respect to the influence of the sun's light on geology； Dr。 Siemens says： 〃The effect of this continuous outpour of solar materials could not be without very important influences as regards the geological conditions of our earth。 Geologists have long acknowledged the difficulty of accounting for the amount of carbonic acid that must have been in our atmosphere at one time or another in order to form with lime those enormous beds of dolomite and limestone of which the crust of our earth is in great measure composed。 It has been calculated that if this carbonic acid had been at one and the same time in our atmosphere it would have caused an elastic pressure fifty times that of our present atmosphere； and if we add the carbonic acid that must have been absorbed in vegetation in order to form our coal…beds we should probably have to double that pressure。 Animal life； of which we had abundant traces in these 'measures；' could not have existed under such conditions； we are almost forced to the conclusion that the carbonic acid must have been derived from an external source。〃 。。。＇
Soon after my paper was read； Lord Murray of Henderland； an old friend； then a Judge on the Scottish Bench； wrote to me as follows： 〃I shall be much obliged to you for a copy； if you have a spare one； of your printed note on Light。 It is expressed with great clearness and brevity。 If you wish to have a quotation for it； you may have recourse to the blind Milton； who has expressed your views in his address to Light：
〃'Hail； holy Light！ offspring of heaven first…born Or of the Eternal co…eternal beam May I express thee unblamed？ since God is light； And never but in unapproached light Dwelt from eternitydwelt then in thee