3° 



NATURE 



[July 6, 191 1 



all the manifold phenomena within the range of our 

 anon. 

 II eight is clearly one of the most significant of these 

 properties. I he eighty or more individual numbers which 

 we call the atomic weights are perhaps the most striking 

 of the physical records nature has given us concerning the 

 earliest stages of the evolution of the universe. They are 

 mute witnesses of the first beginnings of the cosmos out of 

 the chaos, and their significance is one of the first concerns 

 of the chemical philosopher. 



Mankind is not yet in a position to predict any single 

 atomic weight with exactness. Therefore the exact deter- 

 mination of atomic weights rests upon precise laboratory 

 work ; and in order to arrive at the real values of these 

 fundamental constants, chemical methods must be improved 

 and revised so as to free them from systematic or 

 accidental errors. 



What, now, are the most important precautions to be 

 taken in such work? These are worthy of brief notice, 

 because the value of the results inevitably depends upon 

 them. Obvious although they may be, they are olten 

 disregarded. 



In the first place, each portion of substance to be 

 weighed must be free from the suspicion of containing 

 unheeded impurities; otherwise its weight will mean little. 

 This is an end not easily attained, for liquids often attack 

 their containing vessels and absorb gases, crystals include 

 and occlude solvents, precipitates carry down polluting 

 impurities, dried substances cling to water, and solids, 

 even at high temperatures, often fail to discharge their 

 imprisoned contaminations. 



In the next place, after an analysis has once begun, 

 stance into the balance case. 1 Every substance must be 

 collected and find its way in due course to the scale-pan. 

 The trouble here lies in the difficulty in estimating, or 

 even detecting, minute traces of substances remaining in 

 solution, or minute losses by vaporisation at high tempera- 

 tures. 



Iti brief, " the whole truth and nothing but the truth " 

 is the_ aim. The chemical side of the question is far 

 more intricate and uncertain than the physical operation 

 of weighing. For this reason it is neither necessary nor 

 advisable to use extraordinarily large amounts of material ; 

 from 5 to 20 grams in each experiment is usually enough. 

 The exclamation, " What wonderfully fine scales you 

 must have to weigh atoms," simply indicates ignorance; 

 the real difficulties precede the introduction of the sub- 

 stance into the balance case. 1 Every substance must be 

 assumed to be impure, every reaction must be assumed to 

 be incomplete, every measurement must be assumed to 

 contain error, until proof to the contrary can be obtained. 

 Only by means of the utmost care, applied with ever- 

 watchful judgment, may the unexpected snares which 

 always lurk in complicated processes be detected and 

 rendered powerless for evil. 



lh.it the atomic weights may be connected bv precise 

 mathematical equations seems highly probable; but 

 although many interesting attempts have been made to 

 solve the problem, 3 the exact nature of such relationships 

 has not yet been discovered. No attempt which takes 

 liberties with the more certain of the observed values is 

 worthy of much respect. It seems to me that the dis- 

 covery of the ultimate generalisation is not Iikrlv to occur 

 until many. atomic weights have been determined with 

 the greatest accuracy. No trouble being too great to 

 attain this end, the Harvard work will be continued in- 

 definitely, and attempts will be made to improve its 

 quality, for the discovery of an exact mathematical 

 relationship between atomic weights would afford us an 

 immeasurably precious insight into the ultimate nature of 

 things. 



But weight is only one of the fundamental properties of 

 menti Volume is almost, if not quite, .'is important 

 in its own way, although far more variable and confusing. 

 All gases, indeed, approach closely to a simple relationship 

 of volumes, defined by the law of Gay Lussac and the 

 rule of Avogadro, and well known to you all. In the 

 liquid and solid state, however, great irregularities are 



I Richards, "Method* Used in Precis. Chemical Investigation," pub- 

 lished by the Carneirie In«t. of Washington, lot 

 a See especially Rydberg, 1S97, xiv., 66. 



NO. 2175, VOL. 87] 



manifest, and very little system as regards volume is. 

 generally reco 



About twi l\ the study of such small irregu- 



larities as exist .r,r. suspicion of 



a possible cause lot the greater irregularities in liquids 

 and solids.' On applying van der Waals's well-known 

 equation to several gases, in some tentative and un- 

 published computation?, ii seemed clem that the quantity 

 b is not really a constant quantity, but is subject to 

 change under the influence of both pressure and tempera- 

 ture. This conclusion has also been reached independently 

 mi der W-aals himself." But if the quantity b (sup~- 

 posed to be dependent upon the space actually occupied by 

 the molecules) is changeable, are not the molecules them- 

 selves compressible ? " 



The next step in the train of thought is perhaps equally 

 obvious. If changes in the bulk of molecules are to be 

 inferred even from gases, may not the expansion and 

 contraction of solids and liquids afford a much better clue 1 

 to the relative expansion and contraction of these 

 molecules? 



Most physical chemists refer all changes in volume to 

 changes in the extent of the empty space between the 

 molecules. But are there, after all, any such empty 

 spaces in solids and liquids? Solids do not behave as if 

 the atoms were far apart within them ; porosity is often 

 conspicuous by its absence. Take, for instance," the case 

 of glass ; the careful experiments of Landolt on the 

 conservation of weight ' show that glass is highly 

 impermeable to oxygen, nitrogen, and water for long 

 periods. Such porosity as occurs in rigid, compact solids 

 usually permits the passage only of substances which 

 enter into the chemical structure of the solids themselves. 

 Thus nitrogen cannot free itself from imprisonment 

 within hot cupric oxide, although oxygen can esc;; p. , 

 again, water cannot evaporate into even the driest of 

 atmospheres from accidental incarceration in crystals 

 lacking water of crystallisation.' Palladium, on occluding 

 hydrogen, is obliged to expand its bulk in order to make 

 room for even this small addition to its substance. The 

 behaviour of platinum, nickel, and iron is probablv 

 analogous, although less marked. 7 Fused quartz, imper- 

 meable when cold, allows of the passage of helium and 

 hydrogen at high temperatures ; " but most other gases 

 seem to be refused admission, and very many solid sub- 

 stances appear to act as effective barriers to the passage 

 of even hydrogen and helium, especially when cold. In 

 these cases, as in so many others, the so-called " sphere 1 

 of influence " of the atom is the actual boundary by which 

 we know the atom and measure its behaviour. 9 Why not 

 call this the actual bulk of the atom? 



From another point of view, the ordinary conception of 

 a solid has always seemed to me little short of an 

 absurdity. A gas may very properly be imagined with 

 moving particles fat apart; but what could give the 

 rigidity of steel to such an unstable structure? The 

 reasonable conclusion, from all the evidence taken together, 

 seems^ to be that the interstices between atoms in solids 

 and liquids must usually be small even in proportion to 

 the size of the atoms themselves, if, indeed, there are any 

 interstices at all. 



Very direct and convincing evidence of another sort is 

 at hand. The idea that atoms may be compressible 



1 Richards, "The Significance of Changing Atomic Volume." Proc 

 Amer. Acad, 1901, xxxvii., 1 ; iqo2, xxxvii., 3C0 ; iqc:\ xxxviii. 

 1904, xxxix., 581 ; Zcits* physikal, I • i '60, 597; 1903, 



xlii.. 1=9 ; 1904. xlix., is- 



-\a.i der Waals, '.tit ' ihysikal. Chem. t 1 >x%iii 157. His 



earlier publication on tl.i is. w ■ h. Amsterdam* 



1898, xxix., 138) was unknown to me at that time. See also Lewis, Proc. 

 Amer. Acad.. tSon. xxxv . .1. 



;; Van der Waals speak- cautiously, but with some conviction, as to the 

 probable Compressibility of tin mi ilecules on p. 283 of the paper cited above. 



4 H. Landolt, "Uber <iie Frhaltung der Masse Lei Chem. Umwand- 

 Abtutndlung det . ,. \kad .1910. 



r ' Richards, V \em., 1392, i., 196; Proc. Amer. \ 



11., 200. 



« Kaker .in.i \dl.ua I 



" Richards and Behr, Publ 1 arnegie Inst*, ico6. I.xi. 



s lacquerod ami Perroi . 



'•'Since thee idi meed. Barlow and P pi havi brought 



forward much inten rring thesienificai ..-of the \olumes 



.nd lii uids, wlia I' ■ i| [ 1 Ms tl <■ atra tint tie a It ins ate clot el v in 



- 1 lai 1 with.oni anotbi 1 1 1 rai 1 1 Ixxxi* . 1175 : ic.07, xcL, ii;o ; 

 ii., 2308). 



