268 PETER GUTHRIE TAIT 



spectrum analysis, had Stokes given his theory to the world when it first occurred 

 to him." 



The third lecture is devoted to the information which spectrum analysis affords 

 as to the chemical composition of the sun's atmosphere, and its physical condition ; 

 the classification of stars, the constitution of nebulae, and the nature of comets.... 



The remarks on the nebulae and on comets will be read with great avidity ; 

 and, by the majority of readers, with some surprise. For it is stated that the 

 planetary nebulae, " making abstraction of the stellar points, consist of glowing gas." 

 And of comets we find : " There can no longer be any doubt that the nucleus 

 consists, in its inner portions at least, of vapour of some kind, and we must add 

 incandescent vapour..." An ingenious suggestion as to the source of this incan- 

 descence is introduced as the "green-house theory." The nucleus is supposed to be 

 surrounded by an envelope of some kind, transparent to the higher but opaque to 

 the lower forms of radiation. Thus solar heat can get freely at the nucleus, but 

 cannot escape until it has raised the nucleus (in part at least) to incandescence. 

 The coma and tail are formed by the condensation of small quantities of this vapour, 

 so that they are mere mists of excessive tenuity. Herschel's suggestion, that the 

 development of the tail is due to electric repulsion exerted by a charge on the sun, 

 is spoken of with approval ; and the production of the requisite charge of the mist- 

 particles is regarded as a concomitant of condensation. Nothing, however, is said as 

 to the opposite charge which the comet itself must receive, nor of the peculiar 

 effects which would arise from this cause : whether in the form of a modification of 

 the shape of the comet's head, or of a modification of its orbit and period due to a 

 constantly increasing attraction exerted by the sun upon a constantly diminishing 

 mass. 



Of course, if this novel theory can stand the test of a full comparison with 

 facts, it will have established its claim to become part of science. But it is hard to 

 take leave of the simple old ideal comet : the swarm of cosmical brickbats : some- 

 thing imposing because formidable : and to see it replaced by what is, in comparison, 

 a mere phantom, owing its singular appearance to the complexity of the physical 

 properties it possesses and the recondite transformations perpetually taking place in 

 its interior. The old idea of a comet's constitution was not only formidable, but 

 was capable of explaining so much, and of effecting this by means so simple and 

 so natural, that one almost felt it deserved to be well-founded ! The new idea 

 makes it resemble the huge but barely palpable 'Efreet of the Arabian Nights, who 

 could condense himself so as to enter the bottle of brass with the seal of Suleymdn 

 the son of Daood ! 



The following sentences are from Tait's review of the Third Course 

 of Stokes' Burnett Lectures, namely, On the Beneficial Effects of Light 

 (see Nature, June 2, 1887, Vol. xxxvi) : 



This volume completes the course of the First Burnett Lecturer on the New 

 Foundation. We have already (Vol. XXIX, p. 545, and Vol. XXXII, p. 361) noticed 



