31S 



♦ KNOWLEDGE 



[April 17, 1885. 



earth's track iu one place and no more.* Secondly the 

 existence of a radiant point for each system of meteois, this 

 radiant point Ijeing fixed among the stars, proved conchi- 

 aively not only that the paths of the meteors must be 

 parallel, but that the velocities with which the meteors 

 enter our atmosphere must enormously exceed the velocities 

 with which difi'erent parts of the earth's surface are carried 

 round owing to her rotation on her axis, and also such 

 velocities as the earth's attraction can impart to matter 

 approaching her from outside. 



It will be well to examine both these last-named points 

 somewhat attentively, as they involve considerations bear- 

 ing importantly on the discovery which has just been made 

 by Mr. Denning. 



Now, first, as to the velocities with which dilTerent parts 

 of the earth's surface are carried round by her rotational 

 movement. These of cour.se are very small compared with 

 the velocities of meteors travelling from interplanetary 

 space across the earth's orbit. The earth herself travels on 

 her orbit at the rate of 18.', miles per second. Meteors 

 having their aphelia near the orbit of the earth might 

 travel much more slowly than this at the earth's distance. 

 But of such meteors we need take little account ; for far 

 the greater number of meteor systems travel on orbits 

 having a greater mean distance than the earth's, and 

 therefore cross the earth's orbit with greater velocity than 

 that with which the earth herself travels. With an orbit 

 not extending indefinitely beyond the solar system, a 

 flight of meteors would travel at the rate of not more than 

 26 miles per second when at the earth's distance. Taking 

 the average actual velocities of interplanetary meteors at 

 certainly not less than 20 miles per second where they 

 cross the earth's orbit, it is manifest that even the rota- 

 tional velocity of the earth at the equator could but 

 slightly aflfect the apparent direction of meteoric motion. 

 For this velocity is but about 17^ miles per minute, and the 

 assumed mean velocity of meteors, 1,200 miles per minute, 

 is nearly 70 times as great. In latitute 45 north or south 

 the rotational velocity is barely one-hundredth of the 

 average velocity of meteors. Of course an effect must be 

 produced by this velocity in the case of a meteor-shower 

 lasting several hours, Still it must be very slight. 



At first sight it might seem as though the effect of 

 the earth's attraction in deflecting meteors from their 

 course, must be much greater, since the earth can com- 

 municate to a body approaching her under her sole in- 

 fluence from without a velocity of nearly seven miles per 

 second. In reality, however, this cause, like the pre- 

 ceding, can produce but slight effects, — as I shall next 

 proceed to explain. 



(To he coitiimcd.) 



The WasHngtou monumout is re])orted by tlic Scientific American 

 to be in danger of falling, 'i'he area of the foundation, exclusive 

 of a space in the centre not directly loaded, is given as 8,277 square 

 feet, but even with this area the load is said to be 11 tons per 

 square foot without wind. ^Mifinn' i .;,".■ 



Platindh has been discovered in New South Wales in connection 

 with gold in the Ophir district. It has been found in the form of 

 small grains in the Hunter and Macleay districts, and a nugget 

 weighing 2G8 grains was obtained from Wiseman's Creek with 

 alluvial gold. The sand of the sea-coast near Richmond river is 

 also found to contain this precious metal. — Athcnaum. 



* In Guillemin's " Le Ciel," and its English version " The 

 Heavens" (until in revising the work I cut out the passage), the 

 rather absurd mistake was made of presenting the August and 

 November meteor-systems as one system crossing our earth's track 

 iu two places ! A very slight familiarity with even the most 

 elementary principles of orbital motion should have shown this to 

 be impossible. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henry J. Slack, F.G.S., F.R.M.S. 



MOLECULAR MOTION. 



EVERYmicroscopist should be acquainted with the curious 

 facts commonly spoken of in books as " molecular 

 motion," or, by the French, as the " Brownian movements," 

 because the great botanist, Robert Brown, though not its 

 discoverer, first succeeded in calling general attention to it 

 as a phenomenon common in plants, and likewise in water 

 containing minute particles. The writer remembers ouce 

 possessing an old pamphlet sold by a London optician, and 

 giving an account of various experiments with different 

 bodies, including metals, which when sufficiently reduced 

 iu size and suspended in water, were seen under the micro- 

 scope to be in a perpetual fidgetty motion. Those insoluble 

 substances which did not differ much in specific gravity 

 from water were found the easiest with which to display 

 this appearance, and it is very readily seen by just touching 

 a waterdrop on a glass slide with a bit of gamboge, covering 

 with thin glass, and subjecting it to a quarter-inch or higher 

 power. 



Before describing further experiments, it may be as 

 well to point out the objection to the common terms 

 " Molecular " and " Brownian " movements. Formerly, 

 the word molecule meant vaguely any very small mass ; 

 but both physicists and chemists now limit its meaning. 

 The i)hysicist regards it as the unit — the smallest particle 

 of any substance that exhibits force and motion. The 

 chemist means by it the smallest particle of any substance 

 capable of manifesting the properties belonging to that 

 substance. His molecule of sugar is the smallest possible 

 bit of sugar ; but he knows that sugar is a compound of 

 hydrogen, carbon, and oxygen, and that its molecule con- 

 tains many atoms of those substances. If any of the bodies 

 that figure as " simple " in chemical books are really so, 

 then their atom and molecule would be the same ; but if 

 oxygen, for example, should ever be decomposed into two 

 or more distinct substances, the chemists' molecule of that 

 gas would be an aggregate of the molecules of its com- 

 ponents. The smallest particle of any substance to which 

 the term molecule is now applied is inconceivably smaller 

 than any object the microscope can show, and the least 

 visible particle contains millions upon millions of ultimate 

 molecules, far exceeding the power of intellect or imagina- 

 tion to follow. 



Our experiments, then, relate to the movements of 

 microscopic particles in liquids, and it may be broadly 

 affirmed that sufficiently minute particles of most bodies 

 not quickly soluble in water exhibit what look like spon- 

 taneous movements when immersed in that fluid. The best 

 account of the matter known to the writer is contained in 

 a paper of Professor Stanley Jevons in the Quarterly 

 Joiirnal of Science tor April, 1878. He traced the subject 

 back to Stephen Gray, in whose essay, published in the 

 " Philosophical Transactions for 1G9G," the moving particles 

 were spoken of as " animated," and some he considered to 

 be " insects." John Bywater, cited by Jevons, mentioned 

 them in 1819, and rejected the notion of vitality, because 

 he found that, whereas animalcules were killed by boiling, 

 sandstone particles that had just been made red hot 

 moved as well as those that had not been heated at all. 

 Professor Jevons proposed to call the motions pedeiic, or 

 in the singular 2'edesis, from the Greek for leaping or 

 bounding, " this," he says, " being the correct description 

 of the phenomenon when seen in perfection." 



