Amedeo Avogadro

Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto was a great Italian scientist.
Born in a noble ancient family of Piedmont, Amedeo Avogadro was a brilliant student, he graduated in ecclesiastical law very young (20) and began to practice. However, soon after he dedicated himself to the study of physics and mathematics, his preferred sciences, and in 1809 he started teaching them (then calledpositive philosophy) at aliceo (high school) in Vercelli (where his family had some properties).
During this stay in Vercelli he wrote a concise note (memoria) in which he declared the hypothesis of what we now callAvogadro's law:equal volumes of gases, at the same temperature and pressure, contain the same  number of molecules;

This memoriahesenttoDe Lamétherie'sJournaldePhysique,de Chemieetd'Histoirenaturelleanditwas publishedintheeditionofJuly 14, 1811withthetitleEssaid'une manièrededéterminerlesmassesrelativesdesmoléculesélémentairesdescorps,etlesproportionsselonlesquellesellesentrentdanscescombinaisons.
Avogadro'sLaw impliesthattherelationship occurringbetweenthe weightsofsamevolumesofdifferent gases(atthesametemperatureandpressure)correspondstotherelationshipbetweenrespectivemolecular weights.Hence,relativemolecular weightscanbecalculatedfromthe weightsof gases.
Avogadro developedthishypothesisafter Joseph Louis Gay-Lussachad publishedin1808hislawonvolumes(and combining gases).Thegreatestdifficulty Avogadrohadtoresolvewasthehugeconfusionatthattimeregarding atomsand molecules –oneofmostimportantcontributionsof Avogadro'sworkwasclearlydistinguishingonefromtheother, admittingthatsimple particlestoocouldbecomposedof molecules,andthatthesearecomposedof atoms.Forinstance,John Daltondidn'tconsiderthispossibility. Avogadrodidnotactuallyusetheword"atom"asthewords"atom"and"molecule"wereusedalmostwithoutdifference.Heconsideredthattherewerethree kindsof "molecules,"includingan"elementarymolecule"(our"atom").Also,akeenerattentionwasgiventothedefinitionofmass,asdistinguishedfromweight.

In 1814he publishedMémoiresurlesmassesrelativesdesmoléculesdescorpssimples,oudensités présuméesdeleurgaz,etsurlaconstitutiondequelquensdeleurcomposés,pourservirdesuiteàl'Essaisurle mêmesujet,publiédansleJournaldePhysique,juillet 1811aboutgas densities.
In 1820hebecameaprofessorof Turin'suniversity;In 1821he publishedanother memoria,Nouvelles considérationssurlathéoriedesproportionsdéterminéesdanslescombinaisons,etsurla déterminationdesmassesdesmoléculesdescorpsandlittleafterMémoiresurla manièrederamenerles composèsorganiquesauxloisordinairesdesproportionsdéterminées.
Withsuspiciousenthusiasm,hetookpartinpoliticalrevolutionary movementsof 1821(againstthekingofSardinia),sotwoyearslaterhewas removedfromhisposition(or,asitwasofficially declared,theuniversitywasverygladtoallowthisinterestingscientisttotakearestfromheavyteaching duties,inordertobeabletogiveabetterattentiontohis researches).However,overtimethispoliticalisolationwasgraduallyreduced,sincerevolutionary ideaswerereceivingincreasingattention fromSavoy kings,upto 1848whenCharles AlbertgrantedamodernConstitution (Statuto Albertino).Wellbeforethis,followingtheincreasingattentiontohisworks, Avogadrohadbeen recalledat Turinuniversityin 1833,wherehetaughtforanothertwentyyears.
In 1841he completedand publishedhisworkinFisica dei corpi ponderabili, ossia Trattato della costituzione materialede' corpi, 4volumes.
Verylittleisknownabouthisprivatelifeandhispoliticalactivity;despitehisunpleasantaspect(atleastas depictedintherareimagesfound),hewasknownasadiscreettombeurdefemmesalthoughdevotedtoasoberlifeandareligiousman.Hehadsixchildren.Severalhistoricalstudieswouldconfirmthathehad sponsoredand helpedsomeSardinian plotterswhowere organisingarevolutioninthatisland, stoppedattheverylastmomentbytheconcessionofCharles Albert'sstatute.Some doubtshoweverremain,consideringtheverylittleamountofevidence.
Avogadroheldpublic postsinstatistics,meteorology,and weightsand measures(he introduceddecimalmetricsystemin Piedmont)andwasamemberoftheRoyalSuperiorCouncilonPublicInstruction.
Thescientificsocietydidn'treserveagreatattentionatthistheory,so Avogadro'shypothesiswasn'timmediatelyacceptedwhen announced. André-MarieAmpèretoowasablethreeyearslatertoachievethesameresultbyanothermethod(inhisSurla déterminationdesproportionsdanslesquelleslescorpssecombinentd'aprèslenombreetladispositionrespectivedesmoléculesdontleursparticules intégrantessontcomposées),butthesameindifferentregardwasgiventohis theoriesaswell.
Onlywiththestudiesby Gerhardt, Laurentand Williamsononorganicchemistry,wasitpossibletodemonstratethat Avogadro'slawwasindispensabletoexplainwhysame quantitiesof molecules,broughttoavapourstate,havethesamevolume.
Unfortunately,intheperformanceofrelated experiments,someinorganicsubstancesshowedexceptionstothelaw.Thematterwasfinally concludedby Stanislao Cannizzaro,as announcedat KarlsruheCongress (1860,fouryearsafter Avogadro'sdeath),wherehe explainedthattheseexceptions happenedbecauseofmolecular dissociationswhich occurredatcertain temperatures,andthat Avogadro'slawcoulddeterminenotonlymolarmasses,butasaconsequence,atomicmassestoo.
Clausius,byhiskinetictheoryon gaseswasabletogiveanotherconfirmationof Avogadro'slaw.Notlongafter,inhis researchesregardingdilutesolutions(andtheconsequentdiscoveryofanalogiesbetweenthebehaviourofsolutionsand gases), J. H.van't HoffaddedhisfinalconsensusforthetriumphoftheItalianscientist,whosincethenhasbeenconsideredthefounderofthe atomic-moleculartheory.
Inhonourof Avogadro'scontributionstothetheoryof molesandmolecular weights,thenumberof moleculesinonemolewas renamedAvogadro'snumber.

Takenfrom:

http://en.wikipedia.org/wiki/Amedeo_Avogadro

Also see:
http://www.bulldog.u-net.com/avogadro/avoga.html
http://www.chemheritage.org/EducationalServices/chemach/ppt/aa.html