6i6 



NATURE 



[February 4, 19 15 



dust; while if heated by hot air, the air heats 

 the furniture, and in so doing deposits its dust on it. 

 These remarks apply to the fine dust, and not to 

 the larger particles which fall on the furniture, and 

 do not adhere to it like the heat-deposited ones. 

 Electric lighting keeps the ceilings much cleaner 

 than gas. Much of this cleanness is due to the 

 much lower temperature of the air rising from the 

 electric bulb than from gas-lighting, but ceilings 

 over electric lights show blackening, especially in 

 smoking-rooms. 



The cause of the streaks on plaster referred to by 

 Mr. Cope may, however, be a little more complicated 

 than stated above, because the plaster is porous, and 

 some amount of diffusion will take place between 

 the gases in the room and those at the back of the 

 plaster, and as the laths will reduce the diffusion their 

 action will, to some extent, aid their heat-conserving 

 effect. The principal cause of the streaks would, how- 

 ever, appear to be the heat effect, as it will be gener- 

 ajly found that, if the heating and other conditions 

 are the same, the ceilings of the rooms on the top 

 fiat of a house are much more lath and beam marked 

 than those underneath, owing to the upper surface of 

 the plaster in the upper rooms being exposed to the 

 cold air under the slates while the ceilings of the 

 lower rooms are kept warmer by the rooms over 

 them. 



It is possible the difference in the plaster in the 

 cold room referred to by Mr. Cope may be due not 

 to any action of the water vapour, but to its 

 condensation on the walls ingraining the dust into 

 them. 



The reply to Mr. Cope's last question is, yes. A 

 reversal of the phenomenon is quite simple, and has 

 already been referred to. Any surface hotter than 

 the air keeps free of dust ; a surface placed in 

 a smoky chimney, if it is hotter than the gases, 

 gets no soot deposited on it. A paper bearing on 

 the above subject, and entitled "The Formation of 

 Small Clear Spaces in Dusty Air," appeared in the 

 Trans. Roy. Soc, Edin., xxxii., part ii. 



In my letter in Nature of January 21, the date of 

 a letter there referred to is given as March 16, 1913, 

 which should have been March 6, 1913. 



John Aitken. 



Ardenlea, Falkirk, January 26. 



The cause of these streaks, which are also often 

 to be seen on ceilings, appears to be due to the fact 

 that bodies which are warmer than the atmosphere 

 are surrounded by a "dust-free space," and that dust 

 is battered upon surfaces which are cooler than the 

 atmosphere. 



The dust-free space has been described by Tyndall 

 ("Dust and Disease," Royal Inst., 1870), Frank- 

 land ("Dust and Disease," Proc. Rov. Soc, vol. 

 XXV., p. 542), Rayleigh (Roy Soc, December 21, 

 1882; Nature, vol. xxviii., p. 139), Aitken (Roy. Soc. 

 Ed., January 21, 1884) and Lodge and Clark {Vhil. 

 Mag., March, 1884, p. 214). 



Recently I have discussed the question of the 

 discoloration of walls and ceilings (the Engineer, 

 July 3, 1914) in an article on the "Theory of the 

 Radiator." 



In the above papers your correspondent will find 

 answers to the questions he puts. R. M. Deeley. 



Abbeyfield, Salisbury Avenue, Harpenden, 

 January 22. 



Adelard of Bath and the Continuity of Universal 

 Nature. 



In the recently published volume of " Roger Bacon 

 Commemoration Essays " (i) Prof. Pierre Duhem's 

 contribution, " Roger Bacon et I'Horreur du Vide," 



NO. 2^62* VOL. 94I 



has for its main thesis that Bacon was the first to 

 formulate a theory of universal continuity ; an in- 

 correct hypothesis, it is true, but one which Prof. 

 Duhem believes to have served the useful purpose of 

 supplementing "the peripatetic theory of heavy and 

 lip-ht " until the discovery of atmospheric pressure. 

 This theory developed in connection with certain 

 problematical phenomena of which this " experiment " 

 is the chief and typical case. If there is suspended in 

 air a vessel of water having a hole in the top and 

 several narrow apertures in the bottom, no water will 

 fall from it so long as the superior aperture is closed. 

 Yet water is heavier than air, and according to the 

 principle of Aristotle's physics should fall to the 

 ground. Writers before Bacon, according to Duhem, 

 explain this anomaly by saying that the fall of the 

 water would produce a vacuum, and that a vacuum 

 cannot exist m nature. But Bacon argues that a 

 vacuum cannot be the reason why the water does not 

 fall, because a vacuum does not exist; he then ex- 

 plains further that although by their particular natures 

 water tends downwards and air upwards, by their 

 nature as parts of the universe they tend to remain 

 in continuity. Duhem holds that Bacon was the first 

 to substitute this positive law of universal continuity 

 for the mere negation that a vacuum cannot exist in 

 nature (2). 



Prof. Duhem Siupports his case by citation of 

 Greek, Byzantine, and Arabian sources, and by use 

 of writings of fourteenth-century physicists available 

 only in manuscripts. But unfortunately for his main 

 contention he has overlooked that remarkable little 

 treatise, " Questiones naturales," which Adelard of 

 Bath, Bacon's countryman, wrote more than a century 

 before Roger penned his " Questiones naturales " (3). 

 In Adelard 's fifty-eighth chapter his nephew says — the 

 work takes the form of a dialogue between Adelard 

 and his nephew — "There is still one point about the 

 natures of waters which is unclear to me." He then 

 asks his uncle to explain a water jar, similar to that 

 just described, which they had once seen at the house 

 of an enchantress. Adelard replies in his clear, easy 

 style, so different from the scholastic discussion in 

 Bacon's corresponding passages : — 



" If it was magic, the enchantment was worked by 

 violence of nature rather than of waters. For 

 although four elements (4) compose the body of this 

 world of sense, they are so united by natural affection 

 that, as no one of them desires to exist without 

 another, so no place is or can be void of them. There- 

 fore immediately one of them leaves its position 

 another succeeds it without interval, nor can one leave 

 its place unless some other which is especially attached j 

 to it can succeed it." Hence it is futile to give thie 

 water a chance to get out unless you give the air a 

 chance to get in. Finally, Adelard not only thus 

 anticipates the theory of universal continuity, he de- 

 scribes what actually occurs in the "experiment* 

 more accurately than Bacon or the other physicist! 

 cited by Duhem. " Hence it comes about that, if ii 

 a vessel which is absolutely tight above an apertun 

 is made below, the liquid flows out only interruptedly 

 and with bubbling. For as much air gets in a 

 liquid goes out, and this air, since it finds the wate 

 porous, by its own properties of tenuity and lightnes 

 makes its way to the top of the vessel and occupie 

 what seems to be a vacuum " (5). 



Lynn Thorndike. 



Western Reserve University, Cleveland, 

 Ohio, U.S.A. 



(i) Edited by A. G. Little, Oxford, 1914. 



(2) Bacon Essays, p. 266. " Le doctrine dont n« 

 avons suivi le d^veloppement au travers des ^crits^ 

 Roger Bacon semble bien lui appartenir en proj 



