HARDWICKE'S SCIENCE -GOSSIP. 



267 



that over and above the bright line spectrum of 

 which I have so frequently spoken, there exists 

 generally an excessively faint continuous spectrum, 

 such as would be given by a liquid or solid nucleus. 

 No observations have been made of the relative 

 brightness of this continuous spectrum in different 

 nebulae, nor as compared with that of the lines 

 themselves, and in the face of the difficulties to be 

 surmounted in such an inquiry no undertaking of 

 the kind is likely to be attempted. Still, the ex- 

 istence of the continuous spectrum tends very 

 strongly to confirm our supposition, since this is 

 precisely the way in which the product of the 

 chemical union of the cometary matter with the 

 nebular would be expected to show itself. On the 

 contrary, an increased density of the gases near the 

 centre gradually becoming in the more marked 

 instances a liquid nucleus, such as the nebular 

 hypothesis assumes to exist in such cases, would 

 lead to a very different result. If this were the 

 condition of the condensed or developed portion of 

 the nebula, we should necessarily find the bright 

 lines assume a much more sensible width there 

 than near its borders, where the gases would be 

 subject to less pressure, or at least that in different 

 nebulae the bright lines would present considerable 

 differences of width, a result so entirely opposed 

 to observation, that it does not seem too much to 

 say that the theory I have suggested and the hypo- 

 thesis 'of Laplace must stand or fall together, or 

 that the supporters of the latter must suggest some 

 explanation or emendation which at present they 

 have not attempted. 



A few well-developed nebulae should present us 

 with another feature. When all, or nearly all, the 

 original gaseous constituents have been absorbed by 

 combination with other matter, and the nebula has 

 assumed a liquid or semi-solid form, we ought to 

 find a body giving a continuous spectrum, but irre- 

 solvable by the telescope into individual stars ; a 

 disc of greater or less extent should present itself 

 similar in form and appearance to the well-known 

 planetary nebulae. It will be known to my readers 

 that a considerable number of these have equally 

 defied the powers of the telescope and of the spec- 

 troscope. While the former instrument, in its 

 most powerful form, has failed to pronounce them 

 aggregations of stars, the latter has also failed to 

 declare them gaseous. The presumption has hitherto 

 been that this remainder would be resolved into 

 stellar groups if the powers of the telescope could 

 be indefinitely increased ; but this may still be ques- 

 tioned. To assume such developed nebulas to be 

 liquid globes in the second stage of formation would 

 be absurd, didlwe hold that these bodies are more 

 remote than the stars tLemselves ; but there is no 

 evidence whatever that such is the case, and in the 

 want of it we are equally at liberty to believe them, 

 on the contrary, our nearest neighbours. It is an 



assumption also that brings with it no slight sense 

 of relief, since we are no longer obliged to attribute 

 to the larger members of the group dimensions so 

 enormous, that even the mind trained to compre- 

 hend the greatest magnitudes fails and shrinks from 

 attempting to grasp their real volume. 



I now come to speak of the probable uses of 

 comets to systems like our own, which are fully 

 developed, and find myself equally left to speculate 

 rather than to determine, Eor some years the 

 question of the source whence the enormous expen- 

 diture of solar heat is replenished has remained 

 undiscussed and undecided, and yet it is a problem 

 well worthy consideration. If the only satisfactory 

 theory that has been proposed, namely, that the 

 solar waste is renewed by the heat generated by 

 the falling of meteoric bodies upon its surface, is 

 upheld, the intimate connection of these with comets 

 puts the latter in somewhat close relation with this 

 subject. Nor is the popular opinion that large 

 comets give rise to increased solar heat altogether 

 to be despised, especially as it is found to be an 

 almost universal belief. Statistics are not wanting 

 to show that years noted for large comets have also 

 been remarkable for exceptional crops and vintages. 

 In Germany and France the comet wines are inva- 

 riably spoken of as the most excellent, while iu 

 England the popular notion is as well known as it 

 is well supported by several recent and rather per- 

 plexing instances. Wide-spread beliefs such as this 

 are always to be respected, being generally the 

 result of an immense number of individual expe- 

 riences ; and although of themselves of no value as 

 evidence of a scientific truth, they will often direct 

 our investigations aright when we may be wise 

 enough to be guided by them. Schiaparelli has 

 demonstrated that since meteoric streams follow iu 

 the orbits of comets, the latter must be environed 

 by the densest part of the stream, and our expe- 

 riences of the Biela Comet meteors which fell in 

 such profusion on November 27th, 1872, and of 

 those known as the Perseids, and which are con- 

 nected with a comet visible in 1862, fully bear out 

 his deductions. If, therefore, the meteors supply 

 the Waste of solar heat, it is not difficult to under- 

 stand that the approach of a comet, especially when 

 large, accompanied by a dense, wide - stretching 

 cloud of these bodies, may produce a marked in- 

 crease in the amount of heat received from the sun, 

 because a large addition to the ordinary supply of 

 fuel has been brought within the range of its attrac- 

 tion ; * but it must be noted that owing to the 



* If the neighbourhood of the sun is occupied by a very 

 extended atmosphere, the smaller meteorites, as offering 

 relatively to their mass a greater opposing surface, will be 

 more impeded by their passage through it than the larger, 

 and will fall in spiral orbits upon the sun's surface : this 

 would materially increase the sun's range or power of attract- 

 ing these bodies to itself, as I have already hinted in the 

 previous part. 



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