July 4, 1884.] 



KNOWLEDGE ♦ 



to be coQsidered the fact that were it not for the compara- 

 tively small expense involved in taking and 'warehousing 

 electrotypes, many valuable but little saleable works could 

 not bear the cost of a second edition. 



The amateur who aspires to produce good work even- 

 tually is advised to be particular in every detail, and not 

 to rest satisfied until every mould immersed in the bath is 

 efficiently copied. In experimenting in electro metallurgy 

 there is nothing to be apprehended of an unduly difficult or 

 impracticable nature. Cleanliness is essential. Having 

 satisfied himself with his proficiency in copying coins and 

 such-like simple surfaces, the experimentalist may next 

 proceed to more elaborate and more intricate work. To 

 obtain a silvered or gilded copy of the skeleton of a leaf 

 should not be too great a task for him ultimately, and 

 what kind of work is there that is prettier or more 

 interesting ? 



Supposing that, with the aid of the elastic mould, copies 

 have been obtained of medals, >tc., more or less undercut, 

 let us next direct our attention to plaster casts and other 

 similar works of art. Small and simple busts, etc., should 

 be attempted first. An interesting experiment is to coat a 

 small bust with copper. This is easily effected by first 

 saturating the plaster with bee's-wax or linseed oil — the 

 former by preference — and then applying a good coat- 

 ing of plumbago. Some unimportant portion of the bust 

 has then attached to it the wire connected with the zinc 

 pole of the battery. It is not difficult to imagine that any 

 mould which is in deep relief, or considerably undercut, 

 will, under ordinary circumstances, receive a very uneven 

 deposit, that portion which lies in the immediate neighbour- 

 hood of the connecting wire receiving the lion's share. To 

 ob^date this, a few short pieces of fine wire are firmly 

 attached to the wire from the zinc pole of the battery, and 

 their free ends placed in the hollow and more remote parts 

 of the mould. By this means a more eqxiable deposit is 

 ensured. In the present experiment of copper-plating a 

 bust, only a very thin deposit may be permitted, otherwise 

 there will be a considerable want of definition, the finer 

 lines, which are essential to the character of any work of 

 art, being lost. I have had comjiaratively thick deposits 

 in which the design is somewhat faithfully depicted on both 

 sides of the copy, but such a result can only be looked for 

 after considerable practice. As a rule, the thinner the 

 film, the truer will be the design. 



COMET FAMILIES OF THE GIANT 

 PLANETS. 



By Richard A. Proctor, 



THERE is a f\imily of comets every member of which 

 travels iu an orbit passing near the orbit of Jupiter ; 

 another family every member of which can be similarly 

 associated with Saturn ; others depending in the same way 

 on Uranus ; others on K'eptune : and in fact, so fully has 

 this sort of relation been recognised, that the idea has even 

 been thrown out that a planet travelling outside the orbit 

 of Neptune but as yet unknown might be detected by the 

 movements of a comet intersecting the great plane of plane- 

 tary movement far beyond Neptune's orbit. It may be 

 mentioned, indeed, in passing, that the comet of 1862, 

 which has been associated with the meteors of Aug. 10 and 

 11, intersects the plane of planetary movements at a place 

 about as far beyond the orbit of Neptune as that orbit is 

 beyond that of Uranus, and that it has been held probable 

 that at that distance an as yet undiscovered giant planet 

 mav travel. 



This remarkable relation among the orbits of the comets 

 which travel periodically around the sun has been inter- 

 preted by supposing that all such comets were drawn in 

 from outer space by the sun's attraction, and prevented 

 from returning to outer space by the disturbing influence 

 of one or other of the giant planets. If we suppose a 

 comet, drawn sunwards past the orbit of Jupiter, to be so 

 perturbed by the action of that planet as to lose a consider- 

 able portion of its velocity, then that comet would travel 

 thereafter on an orbit passing close to the point on 

 Jupiter's orbit where it had been thus perturbed in such 

 sort as to become an attendant on the sun. But in the 

 first place the explanation requires that the original orbit 

 of the comet should have passed near to the orbit of 

 Jupiter, and a little consideration will show that there 

 should be millions of comets for each thus travelling, — a 

 numerical relation not found to exist among the cometic 

 systems. And secondly, while the explanation would be 

 valid enough were a comet a solid globe or very small, it 

 fails utterly when we recognise that a comet is a flight of 

 bodies occupying a very large extent of space. It can lie 

 shown that supposing a comet's head to be but 10,000 miles 

 in diameter, and formed of discrete meteoric masses, then if 

 the comet came near enough to Jupiter for its centre to be 

 disturbed in the way the theory requires, those meteoric 

 masses nearest to Jupiter would be so much more disturbed 

 as to be sent on very different orbits, while the new orbits 

 of those masses farthest from Jupiter would be so much 

 less disturbed that their orbits would also be entirely diffe- 

 rent. The theory that such comets have been introduced 

 from without fails utterly in the presence of observed facts, 

 and would never indeed have obtained acceptance for an 

 instant but for the carelessness with which such theories 

 are too often dealt with, being presented as abstract ideas 

 instead of being tested in measure and quantity. 



The existence of the comet families of the giant planets 

 can scarcely be explained without assuming, what we have 

 already in another way been led to recognise, — the ejection 

 from the giant planets of masses of matter in eruptions 

 akin to those which take place in the sun. Whether such 

 eruptions take place now in the giant planets or not would 

 be difficult to prove, for although we have evidence of 

 tremendous disturbances, we have nothing to show con- 

 clusively that these would suffice to eject matter for ever 

 from within these planets' globes. Whether a careful study 

 of the region outside the discs of Jupiter and Saturn would 

 reveal aught throwing light on this matter, I am not prepared 

 to say ; but I am certain the edges of the discs of the giant 

 planets are worth much more careful study than they have 

 yet received. Undoubtedly many of the comets of Jupiter's 

 family must have been added to the solar cometic system 

 hundreds of thousands, if not millions, of years ago. But 

 quite possibly both Jupiter and Saturn still eject matter 

 from time to time with such velocities from their interior 

 that it passes away never to return to them. In this as in 

 many other features Jupiter and Saturn resemble the sun. 

 They may be regarded as telling us in some degree what 

 was the past of our own earth, when she was full of the 

 fiery vigour of planetary youth. But they tell us more 

 clearly what will be the future of our sun when the glowing 

 vapor.iising masses now surrounding him have lost their 

 intense lustre, and ceasing to possess his present life-giving 

 qualities he is approaching the condition of dark suns 

 which exist already in immense numbers within our stellar 

 system. 



Erkatum.— In the third line from the bottom of the paragraph 

 on Liniacy Law Reform, on p. 470 of Vol. Y., "inanre" should be 

 "imi:iu;e." 



