What is claimed is: 1. A method of preparing a metal silicide thermoelement for a thermocouple, said method comprising the steps of: providing a silicon base layer extending from a thermocouple measurement junction to a thermocouple reference junction; applying a transition metal silicide film over said base layer between said measurement junction and said reference junction; and heat treating said metal silicide film under an oxidizing gas atmosphere to form a continuous SiO.sub.2 overlayer on said film, said atmosphere having a partial pressure of oxidizing gas sufficient to oxidize silicon atoms from said metal silicide film to SiO.sub.2 but insufficient to oxidize transition metal atoms from said metal silicide film, whereby silicon atoms from said metal silicide film which are oxidized to SiO.sub.2 are replaced in said film by silicon atoms from said silicon base layer. 2. A method according to claim 1, wherein said transition metal silicide is molybdenum silicide or titanium silicide. 3. A method according to claim 1, wherein said metal silicide film is deposited by sputtering a metal silicide target. 4. A method according to claim 1, wherein said metal silicide film is deposited by co-sputtering a transition metal target and a silicon target. 5. A method according to claim 1, wherein said metal silicide film has a thickness of from about 0.1 .mu.m to about 5 .mu.m. 6. A method according to claim 5, wherein said metal silicide film has a thickness of from about 0.5 .mu.m to about 3 .mu.m. 7. A method according to claim 1, wherein said oxidizing gas atmosphere comprises argon or hydrogen and argon containing about 10.sup.-2 atm. partial pressure of water vapor. 8. A method according to claim 1, wherein said heat treating step is carried out at a temperature of from about 800.degree. C. to about 1500.degree. C. for a time from about 1/6 hour to about 3 hours. 9. A method according to claim 8, wherein said heat treating step is carried out at a temperature of from about 900.degree. C. to about 1200.degree. C. for a time from about 1/2 hour to about 2 hours. 10. A method according to claim 1, wherein said silicon base layer is provided by sputter depositing a silicon layer having a thickness of from about 0.3 .mu.m to about 3 .mu.m on an alumina substrate. 11. A high temperature and corrosion resistant thermocouple comprising a first conductive thermoelement extending between a measurement junction and a reference junction and a dissimilar second conductive thermoelement extending between said measurement junction and said reference junction, said first and second thermoelements having different electrical resistivities and being in electrical communication with each other only at said measurement junction and at said reference junction, wherein at least one of said first and second thermoelements comprises a conductive transition metal silicide film disposed on a silicon base layer and covered by an oxygen diffusion limiting SiO.sub.2 overlayer, said at least one thermoelement being stable at temperatures up to at least about 1,000.degree. C. 12. A thermocouple according to claim 11, wherein said transition metal silicide is molybdenum silicide or titanium silicide. 13. A thermocouple according to claim 11, wherein said transition metal silicide film has a thickness of from about 0.1 .mu.m to about 5 .mu.m. 14. A thermocouple according to claim 13, wherein said transition metal silicide film has a thickness of from about 0.5 .mu.m to about 3 .mu.m. 15. A thermocouple according to claim 11, wherein said conductive transition metal silicide film has a resistivity of from 5 .mu..OMEGA.cm to about 500 .mu..OMEGA.cm. 16. A thermocouple according to claim 11, wherein said silicon base layer is formed by sputter depositing a silicon layer on an alumina substrate. 17. A thermocouple according to claim 11, wherein said first thermoelement comprises a conductive film of a transition metal silicide, and second conductive thermoelement comprises a conductive film of a different transition metal silicide. 18. A thermocouple according to claim 11, wherein said first thermoelement comprises a conductive film of a transition metal silicide, and said second thermoelement comprises a film of a conductive metal selected from the group consisting of gold, silver, platinum, palladium, iridium, rhodium and alloys thereof. 19. A thermocouple according to claim 11, wherein said first thermoelement comprises a conductive film of a transition metal silicide, and said second thermoelement comprises conductive substrate, the temperature of which is measured by the thermocouple. 20. A high temperature and corrosion resistant thermocouple comprising a first conductive thermoelement extending between a measurement junction and a reference junction and a dissimilar second conductive thermoelement extending between said measurement junction and said reference junction, said first and second thermoelements having different electrical resistivities and being in electrical communication with each other only at said measurement junction and at said reference junction, wherein at least one of said first and second thermoelements comprises a conductive transition metal silicide film produced by providing a silicon base layer extending from said measurement junction to said reference junction; applying a transition metal silicide film over said base layer between said measurement junction and said reference junction; and heat treating said metal silicide film under an oxidizing gas atmosphere to form a continuous SiO.sub.2 overlayer on said film, said atmosphere having a partial pressure of oxidizing gas sufficient to oxidize silicon atoms from said metal silicide film to SiO.sub.2 but insufficient to oxidize transition metal atoms from said metal silicide film, whereby silicon atoms from said metal silicide film which are oxidized to SiO.sub.2 are replaced in said film by silicon atoms from said silicon base layer.