that鈥攁nd then not on earth but in the sun； where it was found with a spectroscopeduring a solar eclipse； which is why it honors the greek sun god helios。 it wouldn鈥檛 beisolated until 1895。 even so； thanks to mendeleyev鈥檚 invention； chemistry was now on a firmfooting。
for most of us； the periodic table is a thing of beauty in the abstract； but for chemists itestablished an immediate orderliness and clarity that can hardly be overstated。 鈥渨ithout adoubt； the periodic table of the chemical elements is the most elegant organizational chartever devised；鈥潯rote robert e。 krebs in the history and use of our earth鈥檚 chemicalelements； and you can find similar sentiments in virtually every history of chemistry in print。
today we have 鈥�120 or so鈥潯nown elements鈥攏inety…two naturally occurring ones plus acouple of dozen that have been created in labs。 the actual number is slightly contentiousbecause the heavy； synthesized elements exist for only millionths of seconds and chemistssometimes argue over whether they have really been detected or not。 in mendeleyev鈥檚 dayjust sixty…three elements were known； but part of his cleverness was to realize that theelements as then known didn鈥檛 make a plete picture； that many pieces were missing。 histable predicted； with pleasing accuracy； where new elements would slot in when they werefound。
no one knows； incidentally； how high the number of elements might go； though anythingbeyond 168 as an atomic weight is considered 鈥減urely speculative；鈥潯ut what is certain is thatanything that is found will fit neatly into mendeleyev鈥檚 great scheme。
the nineteenth century held one last great surprise for chemists。 it began in 1896 whenhenri becquerel in paris carelessly left a packet of uranium salts on a wrapped photographicplate in a drawer。 when he took the plate out some time later； he was surprised to discoverthat the salts had burned an impression in it； just as if the plate had been exposed to light。 thesalts were emitting rays of some sort。
considering the importance of what he had found； becquerel did a very strange thing： heturned the matter over to a graduate student for investigation。 fortunately the student was arecent 茅migr茅 from poland named marie curie。 working with her new husband； pierre； curiefound that certain kinds of rocks poured out constant and extraordinary amounts of energy；yet without diminishing in size or changing in any detectable way。 what she and her husbandcouldn鈥檛 know鈥攚hat no one could know until einstein explained things the followingdecade鈥攚as that the rocks were converting mass into energy in an exceedingly efficient way。
marie curie dubbed the effect 鈥渞adioactivity。鈥潯n the process of their work； the curies alsofound two new elements鈥攑olonium； which they named after her native country； and radium。
in 1903 the curies and becquerel were jointly awarded the nobel prize in physics。 （mariecurie would win a second prize； in chemistry； in 1911； the only person to win in bothchemistry and physics。）at mcgill university in montreal the young new zealand鈥揵orn ernest rutherford becameinterested in the new radioactive materials。 with a colleague named frederick soddy hediscovered that immense reserves of energy were bound up in these small amounts of matter；and that the radioactive decay of these reserves could account for most of the earth鈥檚 warmth。
they also discovered that radioactive elements decayed into other elements鈥攖hat one dayyou had an atom of uranium； say； and the next you had an atom of lead。 this was trulyextraordinary。 it was alchemy； pure and simple； no one had ever imagined that such a thingcould happen naturally and spontaneously。
ever the pragmatist； rutherford was the first to see that there could be a valuable practicalapplication in this。 he noticed that in any sample of radioactive material； it always took the same amount of time for half the sample to decay鈥攖he celebrated half…life鈥攁nd that thissteady； reliable rate of decay could be used as a kind of clock。 by calculating backwards fromhow much radiation a material had now and how swiftly it was decaying； you could work outits age。 he tested a piece of pitchblende； the principal ore of uranium； and found it to be 700million years old鈥攙ery much older than the age most people were prepared to grant theearth。
in the spring of 1904； rutherford traveled to london to give a lecture at the royalinstitution鈥攖he august organization founded by count von rumford only 105 years before；though that powdery and periwigged age now seemed a distant eon pared with the roll…your…sleeves…up robustness of the late victorians。 rutherford was there to talk about his newdisintegration theory of radioactivity； as part of which he brought out his piece of pitchblende。
tactfully鈥攆or the aging kelvin was present； if not always fully awake鈥攔utherford notedthat kelvin himself had suggested that the discovery of some other source of heat wouldthrow his calculations out。 rutherford had found that other source。 thanks to radioactivity theearth could be鈥攁nd self…evidently was鈥攎uch older than the twenty…four million yearskelvin鈥檚 calculations allowed。
kelvin beamed at rutherford鈥檚 respectful presentation； but was in fact unmoved。 he neveraccepted the revised figures and to his dying day believed his work on the age of the earth hismost astute and important contribution to science鈥攆ar greater than his work onthermodynamics。
as with most scientific revolutions； rutherford鈥檚 new findings were not universallyaccepted。 john joly of dublin strenuously insisted well into the 1930s that the earth was nomore than eighty…nine million years old； and was stopped only then by his own death。 othersbegan to worry that rutherford had now given them too much time。 but even withradiometric dating； as decay measurements became known； it would be decades before we gotwithin a billion years or so of earth鈥檚 actual age。 science was on the right track； but still wayout。
kelvin died in 1907。 that year also saw the death of dmitri mendeleyev。 like kelvin； hisproductive work was far behind him； but his declining years were notably less serene。 as heaged； mendeleyev became increasingly eccentric鈥攈e refused to acknowledge the existenceof radiation or the electron or anything else much that was new鈥攁nd difficult。 his finaldecades were spent mostly storming out of labs and lecture halls all across europe。 in 1955；element 101 was named mendelevium in his honor。 鈥渁ppropriately；鈥潯otes paul strathern； 鈥渋tis an unstable element。鈥�
radiation； of course； went on and on； literally and in ways nobody expected。 in the early1900s pierre curie began to experience clear signs of radiation sickness鈥攏otably dull achesin his bones and chronic feelings of malaise鈥攚hich doubtless would have progressedunpleasantly。 we shall never know for certain because in 1906 he was fatally run over by acarriage while crossing a paris street。
marie curie spent the rest of her life working with distinction in the field； helping to foundthe celebrated radium institute of the university of paris in 1914。 despite her two nobelprizes； she was never elected to the academy of sciences； in large part because after the deathof pierre she conducted an affair with a married physicist that was sufficiently indiscreet toscandalize even the french鈥攐r at least the old men who ran the academy； which is perhapsanother matter。
for a long time it was assumed that anything so miraculously energetic as radioactivitymust be beneficial。 for years； manufacturers of toothpaste and laxatives put radioactivethorium in their products； and at least until the late 1920s the glen springs hotel in the fingerlakes region of new york （and doubtless others as well） featured with pride the therapeuticeffects of its 鈥渞adioactive mineral springs。鈥潯adioactivity wasn鈥檛 banned in consumerproducts until 1938。 by this time it was much too late for madame curie； who died ofleukemia in 1934。 radiation； in fact； is so pernicious and long lasting that even now herpapers from the 1890s鈥攅ven her cookbooks鈥攁re too dangerous to handle。 her lab books arekept in lead…lined boxes； and those who wish to see them must