attached to some grand institution； so he was astounded to discover himteaching elementary arithmetic to boys in a small school on a back street。 according to the scientific historian e。 j。 holmyard； a confused pelletier； upon beholding the great man；stammered：
鈥渆st…ce que j鈥檃i l鈥檋onneur de m鈥檃ddresser 脿 monsieur dalton？鈥潯or he couldhardly believe his eyes that this was the chemist of european fame； teaching a boyhis first four rules。 鈥測es；鈥潯aid the matter…of…fact quaker。 鈥渨ilt thou sit downwhilst i put this lad right about his arithmetic？鈥�
although dalton tried to avoid all honors； he was elected to the royal society against hiswishes； showered with medals； and given a handsome government pension。 when he died in1844； forty thousand people viewed the coffin； and the funeral cortege stretched for twomiles。 his entry in the dictionary of national biography is one of the longest； rivaled inlength only by those of darwin and lyell among nineteenth…century men of science。
for a century after dalton made his proposal； it remained entirely hypothetical； and a feweminent scientists鈥攏otably the viennese physicist ernst mach； for whom is named the speedof sound鈥攄oubted the existence of atoms at all。 鈥渁toms cannot be perceived by the senses 。 。
。 they are things of thought；鈥潯e wrote。 the existence of atoms was so doubtfully held in thegerman…speaking world in particular that it was said to have played a part in the suicide of thegreat theoretical physicist； and atomic enthusiast； ludwig boltzmann in 1906。
it was einstein who provided the first incontrovertible evidence of atoms鈥櫋xistence withhis paper on brownian motion in 1905； but this attracted little attention and in any caseeinstein was soon to bee consumed with his work on general relativity。 so the first realhero of the atomic age； if not the first personage on the scene； was ernest rutherford。
rutherford was born in 1871 in the 鈥渂ack blocks鈥潯f new zealand to parents who hademigrated from scotland to raise a little flax and a lot of children （to paraphrase stevenweinberg）。 growing up in a remote part of a remote country； he was about as far from themainstream of science as it was possible to be； but in 1895 he won a scholarship that took himto the cavendish laboratory at cambridge university； which was about to bee the hottestplace in the world to do physics。
physicists are notoriously scornful of scientists from other fields。 when the wife of thegreat austrian physicist wolfgang pauli left him for a chemist； he was staggered withdisbelief。 鈥渉ad she taken a bullfighter i would have understood；鈥潯e remarked in wonder to afriend。 鈥渂ut a chemist 。 。 。鈥�
it was a feeling rutherford would have understood。 鈥渁ll science is either physics or stampcollecting；鈥潯e once said； in a line that has been used many times since。 there is a certainengaging irony therefore that when he won the nobel prize in 1908； it was in chemistry； notphysics。
rutherford was a lucky man鈥攍ucky to be a genius； but even luckier to live at a time whenphysics and chemistry were so exciting and so patible （his own sentimentsnotwithstanding）。 never again would they quite so fortably overlap。
for all his success； rutherford was not an especially brilliant man and was actually prettyterrible at mathematics。 often during lectures he would get so lost in his own equations thathe would give up halfway through and tell the students to work it out for themselves。
according to his longtime colleague james chadwick； discoverer of the neutron； he wasn鈥檛even particularly clever at experimentation。 he was simply tenacious and open…minded。 forbrilliance he substituted shrewdness and a kind of daring。 his mind； in the words of onebiographer； was 鈥渁lways operating out towards the frontiers； as far as he could see； and thatwas a great deal further than most other men。鈥潯onfronted with an intractable problem； hewas prepared to work at it harder and longer than most people and to be more receptive tounorthodox explanations。 his greatest breakthrough came because he was prepared to spendimmensely tedious hours sitting at a screen counting alpha particle scintillations； as they wereknown鈥攖he sort of work that would normally have been farmed out。 he was one of the firstto see鈥攑ossibly the very first鈥攖hat the power inherent in the atom could； if harnessed； makebombs powerful enough to 鈥渕ake this old world vanish in smoke。鈥�
physically he was big and booming； with a voice that made the timid shrink。 once whentold that rutherford was about to make a radio broadcast across the atlantic； a colleague drilyasked： 鈥渨hy use radio？鈥潯e also had a huge amount of good…natured confidence。 whensomeone remarked to him that he seemed always to be at the crest of a wave； he responded；鈥渨ell； after all； i made the wave； didn鈥檛 i？鈥潯。 p。 snow recalled how once in a cambridgetailor鈥檚 he overheard rutherford remark： 鈥渆very day i grow in girth。 and in mentality。鈥�
but both girth and fame were far ahead of him in 1895 when he fetched up at thecavendish。
1it was a singularly eventful period in science。 in the year of his arrival incambridge； wilhelm roentgen discovered x rays at the university of w眉rzburg in germany；and the next year henri becquerel discovered radioactivity。 and the cavendish itself wasabout to embark on a long period of greatness。 in 1897； j。 j。 thomson and colleagues woulddiscover the electron there； in 1911 c。 t。 r。 wilson would produce the first particle detectorthere （as we shall see）； and in 1932 james chadwick would discover the neutron there。
further still in the future； james watson and francis crick would discover the structure ofdna at the cavendish in 1953。
in the beginning rutherford worked on radio waves； and with some distinction鈥攈emanaged to transmit a crisp signal more than a mile； a very reasonable achievement for thetime鈥攂ut gave it up when he was persuaded by a senior colleague that radio had little future。
on the whole； however； rutherford didn鈥檛 thrive at the cavendish。 after three years there；feeling he was going nowhere； he took a post at mcgill university in montreal； and there hebegan his long and steady rise to greatness。 by the time he received his nobel prize （for鈥渋nvestigations into the disintegration of the elements； and the chemistry of radioactivesubstances；鈥潯ccording to the official citation） he had moved on to manchester university；and it was there； in fact； that he would do his most important work in determining thestructure and nature of the atom。
1the name es from the same cavendishes who producec henry。 this one was william cavendish； seventhduke of devonshire； who was a gifted mathematician and steel baron in victoriar england。 in 1870； he gave theuniversity 锟�6；300 to build an experimental lab。
by the early twentieth century it was known that atoms were made of parts鈥攖homson鈥檚discovery of the electron had established that鈥攂ut it wasn鈥檛 known how many parts therewere or how they fit together or what shape they took。 some physicists thought that atomsmight be cube shaped； because cubes can be packed together so neatly without any wastedspace。 the more general view； however； was that an atom was more like a currant bun or aplum pudding： a dense； solid object that carried a positive charge but that was studded withnegatively charged electrons； like the currants in a currant bun。
in 1910； rutherford （assisted by his student hans geiger； who would later invent theradiation detector that bears his name） fired ionized helium atoms； or alpha particles； at asheet of gold foil。
2to rutherford鈥檚 astonishment； some of the particles bounced back。 it wasas if； he said； he had fired a fifteen…inch shell at a sheet of paper and it rebounded into his lap。
this was just not supposed to happen。 after considerable reflection he realized there could beonly one possible explanation： the particles that bounced back were striking something smalland dense at the heart of the atom； while the other particles sailed through unimpeded。 anatom； rutherford realized； was mostly empty space； with a very dense nucleus at the center。
this was a most gratifying discovery； but it presented one immediate problem。 by all the lawsof conventional physics； atoms shouldn鈥檛 therefore exist。
let us pause for a moment and consider the structure of the atom as we know it now。 everyatom is made from three kinds of elementary particles： protons； which have a positiveelectrical charge； electrons； which have a negative electrical charge； and neutrons； which haveno charge。 protons and neutrons are packed into the nucleus； while electrons spin aroundoutside。 the number of protons is what gives an atom its chemical identity。 an atom with oneproton is an atom of hydrogen； one with two protons is helium； with three protons is lithium；and so on up the scale。 each time you add a proton you get a new element。 （because thenumber of protons in an atom is always balanced by an equal number of electrons； you willsometimes see it written that it is the number of electrons that defines an element； it es tothe same thing。 the way it was explained to me is that protons give an atom its identity；electrons its personality。）neutrons don鈥檛 influence an atom鈥檚 identity； but they do add to its mass。 the number ofneutrons is generally about the same as the number of protons； but they can vary up and downslightly。 add a neutron or two and you get an isotope。 the terms you hear in reference todating techniques in archeology refer to isotopes鈥攃arbon…14； for instance； which is an atomof carbon with six protons and eight neutrons （the fourteen being the sum of the two）。
neutrons and protons occupy the atom鈥檚 nucleus。 the nucleus of an atom is tiny鈥攐nly onemillionth of a billionth of the full volume of the atom鈥攂ut fantastically dense； since itcontains virtually all the atom鈥檚 mass。 as cropper has put it； if an atom were expanded to thesize of a cathedral； the nucleus would be only about the size of a fly鈥攂ut a fly manythousands of times heavier than the cathedral。 it was this spaciousness鈥攖his resounding；unexpected roominess鈥攖hat had rutherford scratching his head in 1910。
it is still a fairly astound