44 



NATURE 



[May 9, 1878 



that the sun is only once that of water. What the sun is 

 he could not tell, biit it is a very poor light creature 

 indeed. The density of Mercury is perhaps rather greater 

 than that of the earth. The density of Venus is much 

 the same as that of the earth, and the density of Mars is 

 also much the same as that of the earth. Then after 

 that comes a shower of little planets, about 200 of 

 which have been observed up to the present time, and 

 he could not tell Avhat they are made of. Then there 

 are Jupiter and Saturn, which are no heavier than 

 water. So that it appears clear that, assuming the forma- 

 tion of these things by the condensation of nebulae, on the 

 theory he had mentioned, the different parts of the 

 nebulae which have contributed to the solar system are 

 very different. Well, that being considered as established, 

 it follows that in the constitution of our earth there may 

 be parts of very different density. He should say that 

 the high and prominent parts of the land are made of 

 something light, and the heavy and dense parts are those 

 covered by a considerable quantity of water, which have 

 sunk deep into the central lava on which, he conceived, 

 all things are resting. 



And now he had come pretty nearly to the end of his 

 theory, and he would show them what he feared they 

 would call an absurd representation of what he conceived 

 the state of the earth to be. [The lecturer drew atten- 

 tion to a diagram of an "ideal earth," roughly show- 

 ing his theory — some parts of the crust of the earth 

 being thick and coloured darkly to indicate density ; 

 some thick and not so dense, and all admitting of vol- 

 canic eruptions from the interior, which was represented 

 as lava.] Remember that everything here is exaggerated. 

 It is not intended to be a correct representation. It is a 

 caricature of the most extravagant kind ; but if it con- 

 veyed to them the broad ideas that had impressed them- 

 selves upon his mind, it would be doing the right thing. 

 He thought a large proportion of the centre of the earth 

 is fluid and hot, and he thought that upon this there were 

 certain divers classes of something like solid matter. In 

 all these parts there are cracks or chinks through which 

 volcanoes burst out where the cover of the earth is very 

 thin. In some places you have two or three volcanoes 

 together. There is one instance in Europe, where we 

 hare Etna, Stromboli, and Vesuvius. In this diagram 

 he had condensed to the best of his conjectural power his 

 supposition as to what the state of the earth really is ; 

 and if any one chose to find fault with it he would not 

 quarrel with him. He only gave it as a sort of inference 

 from a number of things he had said. 



A NEW INSULATING STAND ^ 



CIR WILLIAM THOMSON has frequently dwelt on 

 *^ the great importance of insulating, with the utmost 

 care, any apparatus intended fo;r researches relating to 

 static electricity ; he has shown that the atmosphere and 

 other gases have but little effect in dissipating an electric 

 charge, even when moist, and that it escapes mainly in 

 consequence of the deposition of a layer of moisture upon 

 the insulating supports which renders their surface con- 

 ducting. In all Sir W. Thomson's electrometers there is 

 an arrangement for drying the insulating surfaces by 

 means of sulphuric acid, either free or absorbed by 

 pumice. This method admits of very general applica- 

 tion : — Any body, as for example apparatus constructed 

 for the observation of atmospheric electricity, may be 

 most perfectly insulated by supporting it on glass rods 

 inserted in glass cylinders containing free sulphuric acid 

 or pumice moistened with it. In order to do this the 

 lower end of the rods must be either inserted into 

 cylinders of lead or else fixed to the bottom of the jar by 

 means of a substance not acted upon by sulphuric acid, 

 for example, meked sulphur or paraffin ; melted sulphur is 

 liable, on account of its temperature, to crack the jars, 



' By M. E. Mascart, Professor of Physics, College de France, Paris. 



notwithstanding the precaution of previous heating* 

 paraffin, on the other hand, softens in the course of time> 

 and the glass rods do not retain their vertical position- 

 Notwithstanding these disadvantages excellent insulators 

 may be thus extemporised as occasion may require. 



For permanent use it is advantageous to employ insu- 

 lators specially constructed, as shown in the accompanying 

 figure; it consists of a bottle having a narrow neck, 



through which passes a tubular continuation of the 

 Ijottom, about 4 mm. less in diameter than the internal 

 diameter of the neck, so as to leave a space of 2 mm. 

 (about) between them. The top of this hollow rod is 

 closed, in order that a brass tube may be cemented upon 

 it, into which may be screwed any apparatus, as, for 

 example, a disc as shown in the figure, a sphere, a crutch 

 on a hoop, &c., &c. In the shoulder of the bottle is a 

 neck, closed with a ground-glass stopper, through which 

 sulphuric acid may be poured, in the first instance, and 

 renewed from time to time. As the space between the 

 hollow rod and the neck of the bottle is very small, the 

 air in the bottle does not change very rapidly, and the 

 sulphuric acid remains efficient for a long time. It is 

 only necessary to run off a portion of it occasionally by 

 means of a siphon, and to add fresh ; as this may be 

 done without disturbing the apparatus, the insulation may 

 be maintained for any length of time. Moreover, for an 

 insulator to be used occasionally, an addition is made of 

 a vulcanised rubber cap, which slides on the glass rod to 

 close the neck of the bottle when not in use.^ 



A double pendulum of pith balls supported by such an 

 apparatus maintains its divergence, after being charged 

 with electricity, for a very long time, even in a theatre filled 

 with an audience. One may show by a simple experiment 

 the great efficacy of this apparatus in comparison with 

 insulators of glass exposed to the air, even when carefully 

 varnished with shellac. If a pair of pith balls, suspended 

 by a thread of cotton, is hung upon the latter support, and 

 the metallic foot is placed on an insulator, and connected 

 with a charged condenser, no divergence of the pith balls 

 occurs in the first instance, but little by little the elec- 

 tricity is propagated along the glass rod, and then the 

 threads near the support begin to separate, and soon after 

 the balls diverge and remain at a certain distance from 

 each other. 



The electrometers of Sir Wm. Thomson are sometimes 

 so perfectly insulated that the loss of a charge of elec- 

 tricity does not amount to y^oth part in twenty-four hours. 

 By means of the insulator described above, one may 

 obtain an insulation of like order for bodies supported m 

 the open air, and thus diminish to a great extent one of 

 the chief sources of error usually met with in experi- 

 menting with static electricity. 



^ These may be obtained of various sizes, one litre, half -litre, quarter- 

 litre capacity, at Alvergniat Freres, lo, Rue de la Sorbonne, Pans. 



