May 28, 1874] 



NATURE 



69 



ATOMS AND MOLECULES SPECTRO- 

 SCOPIC ALLY CONSIDERED* 

 T ET me commence by congratulating you on the circumstance 

 that this School and the Literary Society connected with it 

 are known over a much more extensive area than Whitechapel. 

 It is some time ago since I first heard of the work which you 

 are attempting to do, and which indeed to a large extent you are 

 doing, in this part of London. All friends of Science must 

 deeply sympathise with your efforts, and I looked upon it as 

 my bounden duty to come here and lecture when asked to do 

 so. I have one more remark to oflor : as I knew tliat my 

 audience would consist if not altogether of old students of Science 

 in this school, still of those largely interested in mental culture 

 and in tlie acquisition of u--cful knowledge, I thought it right to 

 ask you to follow me into a region which a few men in lands far 

 apart are now investigating — a region which lies outside the 

 known, and which is being explored by means of the spectro- 

 scope. I hope to be able to suggest a few thoughts to some of 

 you, in case you have worked -with that instrument, and I hope 

 also to be able to place before those who have not, many facts 

 with which tliey are already acquaiated, in a new point of view. 



Now, in tire first place, wliat are Atoms and what are Mole- 

 cules ? A chemist will tell you about the atomic weight of certain 

 elements, and you will hear him talk about molecular volumes, 

 and the like. Here is a definition given by Dr. Frankland in 

 his book on Chemistry ("Lecture Notes," p. 2): "An atom is the 

 smallest proportion by weight in which tlie element (that is to 

 say the element to which the atom under discussion belongs) 

 enters into or is expelled from a chemical compound." He 

 then points out that when atoms are isolated — that is, when 

 they are separated from other kinds of matter — they do not 

 necessarily exist as atoms in the old sense ; they go about in 

 company, generally being associated in jiairs. He then defines 

 such a combination of atoms as an elementary molecule. Here, 

 then, is put before us authoritatively a chemist's view of the 

 difference between an atom and a molecule. 



Let us now go to the physicist and see if we can gather from 

 him his idea of atoms or molecules. It is remarkable that, in 

 a most admirable book, Prof Clerk-Maxwell's "Theory of 

 Heat," in which you find nearly all that is known by phybicists 

 about molecular theories, the word "atom" is not used at all. 

 We are at once introduced to the word " molecule," which is de- 

 fined to be "a small mass of matter the parts of vrhich do not 

 part company during the excursions which the molecide makes 

 when the body to which it belongs is hot." 



Prof Clerk-Maxwell goes on to give us ideas about these 

 "molecules," which have resulted from the investigations of him- 

 self and others ; and if yeu will allow me, I will read a few extracts 

 from his book (p. 2S6> : "All bodies consist of a number of 

 small parts called molecules. Every molecule consists of a 

 defmitc quantity of matter, which is exactly the same for all 

 tlie molecules of the same substance. The mode in which tlie 

 molecule is bound together is the same for all molecules of the 

 same substance. A molecule may consist of several distinct 

 portions of matter held together by chemical bond.s, and m.ay be 

 set in vibr.ation, rotation, or any other kind of relative motion, 

 but so long as the different portions ia not part company but 

 travel together in the excursions made by the molecule, our 

 theory calls the whole connected mass a single molecule." Here, 

 then, wc have our definition of a molecule enlarged. Tne 

 next point insisted upon by our author is that the molecules of all 

 bodies lire in a state of contlniinl agitation. 



That this agitation or motion exists in the smallest parts of 

 bodies is partly made clear by the fact that we cannot see the 

 bodies themselves move. 



Now in a solid body the molecule never gets beyond a certain 

 distance from its initial position. The path it describes is often 

 within a very small region of space. Prof. Clifford, in a lecture 

 upon atoms, lias illustrated this very clearly. He supposes a 

 body in the middle of the room held by elastic bands to the 

 ceiling and the floor, and in the same manner to each side of 

 the room. Now pull the body from its place ; it will vibrate, 

 but always about a mem position ; it will not travel boJily out 

 of its place. It will always go back again. 



We next come to fluids : concerning these we read — 

 " In fluids, on the other hand, there is no such restriction to the 



* Revised from short-hand notes of a Lecture delivered to the White- 

 chapel Foundation School Literary and Scientific Society, March jo, 

 1874. 



excursions of a molecule. It is true that th.e molecule generally 

 can travel but a very small distance before its path is disturbed 

 by an encounter with soaie other molecule ; but after this encoun- 

 ter there is nothing wliich determines the molecule rather to 

 return towards the place from whence it came than to push its 

 way into new regions. Heace in fluids the path of a molecule is 

 not confined within a limited region, as in the case of solids, but 

 may penetrate to any part of the space occupied by the fluid." 



Now we have the motion of the molecule in the solid and the 

 fluid. How about the movement in a gas ? " A gaseous body is 

 supposed to consist of a large number of molecules moving very 

 rapidly." For instance, in this room the molecules of the air 

 are travelling about twenty miles in a minute. " During the 

 greater part of their course tliese molecules are not actad upon 

 by any sensible force, and therefore move in straight lines with 

 uniform velocity. When two molecules come within a certain 

 distance of each other, a mutual action takes place between 

 them which may be compared to the collision of two billiard 

 balls. Each molecule has its course changed and starts in a 

 new path." 



The collision between two molecules is defined as an " En- 

 counter ; " the course of a molecule between encounters a " Free 

 path." It is then pointed out that "in ordin.ary gases the free 

 motion of a molecule takes up much more time than is occupied 

 by an encounter. As the density of the gas increases the free 

 path diminishes, and in liquids no part of the course of a molecule 

 can be spoken of as its free path." 



Now the kinetic theory of gases, on which theory these state- 

 ments are made, has this great advantage about it, that it explains 

 certain facts which had been got at experimentally, facts which 

 had been established over and over again, but whicli lacked ex- 

 planation altogether, till this molecular theory, whicli takes for 

 granted the existence of certain small things which are moving 

 rapidly in gases, less r,ipid!y in fluids, and s'iU less in solids, was 

 launched. The theory in fact explains in a most ample manner, 

 many phenomena so well known, that are termed "laws." It 

 explains Boyle's law, and others, well known to students of this 

 school. This theory, which takes for its basis the existence of 

 molecules and their motions, explains pressure by likening it to 

 the bombardment of tlie sides of the containing vessel by the 

 molecules in motion ; or it tells us that the temperature of a gas 

 depends upon the velocity of the agitation of the molecules, and 

 that this velocity of the molecules in the same gas is the same for 

 the same temperature, whatever be the density. When the 

 density varies, the pressure varies in the same proportion. This 

 is Boyle's law. Further, the densities of two gases at the same 

 temperature and pressure are proportional to the masses of their 

 individual molecules, or, when two gases are at the same pressure 

 and temperature, the number of molecules in unit of volume is 

 the same. This is the law of Gay Lussac. 



I have now fairly introduced you to the atom of the chemist 

 and the molecule of the physicist ; you will see at once that the 

 methods of study employed by chemical and physical inves- 

 tigators are widely different. The chemist never thinks about 

 encounters, and the physicist is careless as to atomic weight ; in 

 his mind's eye he sees a perpetual clashing and rushing of par- 

 ticles of matter, and he deals rather with the quality of the 

 various mo'.ions than of the material. 



Next let me say a litle more about these "encounters ; " and 

 here I must again refer you to Prof. Clerk-Maxwell's book 

 (p. 306). It is assumed that while the molecule is traversing 

 its free path after an encounter, it vibrates according to its own 

 law, the law being determined by the construction of the 

 molecule, or let us say its chemical nature, so that the vibration 

 of one particle of sodium would be like that of another par- 

 ticle of sodium, but unlike that of a particle of another chemical 

 substance, let us say iron. If the interval between encounters 

 is long, the molecule may have used up its vibrations before the 

 second encounter, and may not vibrate at all for a certain time 

 previous to it. The amplitude of the vibration will depend 

 upon the kind of encounter, and will be independent of the 

 number of encounters. 



We can imagine a small number of feeble encounters, a large 

 number of feeble encounters, a small number of strong encoun- 

 ters, and a large number of strong encounters. 



In the case of feeble encounters, we pass from a small num- 

 ber to a large one by increasing the density. 



In the case of strong encounters we pass from low temperature 

 with small density to high temperature with great density. 



Increase of density will reduce "free path." 



