46 Obfervations on Platinva, 
were préefented to.me by the minifter of the marine, oft 
hearing an account given of the utility of my refearches, 
have greatly contributed to their fuccefs. Opticians who 
ftudy the nature of thofe fubftances ufed in the compofition 
6f fpecula, know that no metal or femi-metal can fupply 
the place of tin. It is tin which renders red copper brittle, 
elaflic, and fufceptible of a fine polith. This metal produces 
alfo the fame effets on all other metallic fubftances.  Pla- 
tina is fabjected to the fame law. But I fhall not here relate 
the numerous trials I was obliged to make before I could 
obtaim a fatisfaGtory refult. F fhall confine myfelf to an ac- 
count of that procefs, which was attended with the greateft 
faccefs. . 
‘The platina in grains muft be purified im a ftrong fire by 
means of nitre and the falt of olafs*. To the platina, when 
purified, add the eighth part of that metal employed in the 
éompofition of common fpecula ; for tin without red copper 
would not produce a good effect. This mixture is then to be 
expofed to the moft violent heat, which mutt be ftill excited 
by the oxygen gas that difengages itfelf from nitre when 
thrown into the fire. One melting would be mmfufficient ¢ 
five.or fix are requifite to bring the mixture to perfection, 
It is neceflary that the metal fhould be in a ftate of complete 
fufion at the moment when it is poured into the mould. By 
this procefs I have been enabled to conftruét a telefcope with 
platina, which magnifies the diameters of objeéts five hun- 
dred times, with a degree of clearnefs and diftinétnefs requi- 
fite for the niceft obfervations. The large fpeculum of pla- 
tina weighs fourteen pounds: it is eight inches in diameter, 
and its focus is fix feet. This fpeculum, which is unchange~ 
able, was eaft and polifhed by Carrochez. 
The utility of platina in the urts is not confined to the 
conftruction of telefcopes and nautical inftruments. Robin, 
a celebrated watch-maker, wifhed to employ it in making 
* The flux ufed in the Englifh glafs-houfes, and called . by the work- 
men fandifer. Epir. 
for 
