NaturalLibertyRequiresAdherenceto Truth 323understanding.Whathappenedwasas follows.As physicistand sciencehistorianDavid
Harrimannarrates,the chemistsof the Enlightenmenthad to demarcatethe “cleardistinc-
tionbetweenpuresubstancesand mixtures.Unlikemixtures,puresubstanceshavewell-
definedandinvariantproperties.Underthe sameconditions,everysampleof a pure
substancewill melt(or boil)at preciselythe sametemperature.Everysuchsamplehas the
samehardnessandthe samemassdensity,andthe sameamountof heatwillalways
causethe samerise in temperature(for a unitmass).Furthermore,whena portionof a
substanceundergoesa chemicalreaction,the propertiesof the remainingpartdo not
change.” Conversely,the “concept‘mixture’ couldthenbe definedas a materialcom-
posedof two or moresubstances.”
The“nextkey step,” Harrimancontinues,“wasthe divisionof substancesintoele-
mentsand compounds.Thiswasmadepossibleby the discoverythatmassis conserved
in chemicalreactions,i.e. the weightsof the reactantsare alwaysequalto the weightsof
the products.”^11 AntoineLavoisier(1743–1794)^12 provedthis.He constructeda glasscon-
tainerwithweighingscaleson the bottom.He placeda chunkof metalinsidethe contain-
er andthenmeasuredthe weightof boththe metalandthe air aroundit. Whenhe
stimulatedchemicalreactionson the metalso that it wouldrust,the rustingeffectcaused
the metalto gainin weight.The weightthatthe metalgainedwasexactlythe weightlost
to the air aroundthe metal.At boththe beginningandendof the experiment,the total
weightof boththe metalpieceand the air aroundit remainedunchanged.Thisproved(1)
thatmatterchangesformwhile(2) the totalquantityof matterin the universedoesnot
change.^13 Afterthis,Harrimanadds,it “wasfoundthatsomesubstancescan be decom-
posedintotwoor moreothersubstanceswiththe sametotalweight;on the otherhand,
othersubstancesresistedall suchattemptsat chemicaldecomposition.Thosethatcannot
be decomposedare elements.. .” By contrast,thosethatcan be decomposedintotwo or
moreseparateelementsare compounds.ThenJosephLouisProust(1754–1826)demon-
stratedthatcoppercarbonateinvariablyconsistsof copper,oxygen,andcarbonin the
samemassratio(5:4:1).He demonstratedthatthis principleapplieshoweverthe sample
waspreparedand no matterhowit wasisolatedfromthe wildernessenvironment.From
this empiricalobservation,Proustwasableto deviseby 1777the Lawof ConstantCom-
position,whichis that separatesamplesof a chemicalcompoundinvariablyconsistof the
samechemicalelementsin the sameproportion,mass-wise.Thisdiscoveryprovedessen-
tial to the eventualrealizationthateverychemicalelementis comprisedof microscopic
particlesknownas atoms,eachof theseatomsbeingof the sameelement.It waslikewise
essentialto the discoverythat a chemicalcompoundlike coppercarbonateis comprisedof
microscopicparticlesknownas molecules,witheachmoleculebeingcomprisedof at least
two smallerparticles,eachparticlebeingan atom.
In 1803,JohnDaltoncontributedto furtherunderstandingof the atomictheory.As
DavidHarrimannarrates,that was the yearthat Dalton
analyzedthreegasescomposedof nitrogenandoxygen.Thefirstis a colorlessgas that
has a pleasantodorandthe capacityto causehystericallaughterwheninhaled;the
secondis colorless,nearlyodorless,and has a significantlylowermassdensity;the third
has the highestmassdensityof the threeanda deepbrowncolorat hightemperatures.
Quantitativeanalysisshowedthatthe threegasesare alsodistinguishedby the relative
weightsof the twocombiningelements.If we considersamplesof eachgas thatcontain
1.75gramsof nitrogen,thenwe findthatthe laughinggas containsone gramof oxygen,
the secondgas containstwo grams,and the thirdgas containsfourgrams.Daltonfounda
similarresultwhenhe analyzedtwodifferentgasesthatare bothcomposedof carbon
and hydrogen;for samplescontainingthe sameweightof carbon,the weightof hydrogen
containedin one of the gasesis preciselytwicethat of the other.On the basisof suchdata,
Daltonarrivedat a newlaw:Whentwo elementscombineto formmorethanone compound,the