The purpose of this paper is to give a new proof of the Joyal-Tierney theorem (unpublished), which asserts that a morphism $f:R\rightarrow S$ of commutative rings is an effective descent morphism for modules if and only if $f$ is pure as a morphism of $R$-modules.
Keywords: Pure morphisms,(effective) Descent morphisms, Split coequalizers.
2000 MSC: 13C99,18A20,18A30,18A40.
Theory and Applications of Categories, Vol. 7, 2000, No. 3, pp 38-42.
http://www.tac.mta.ca/tac/volumes/7/n3/n3.dvi
http://www.tac.mta.ca/tac/volumes/7/n3/n3.ps
http://www.tac.mta.ca/tac/volumes/7/n3/n3.pdf
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/7/n3/n3.dvi
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/7/n3/n3.ps