132 EFFECTS of HEAT 
I have exerted myfelf to remove, by frefh experiments, every 
doubt that has arifen on the fubjeét. In order to guard againft 
natural impurities, I have applied to fuch of my friends as have - 
turned their attention to chemical analyfis, (a branch of the 
fcience to which I have never attended,) to furnith me with 
carbonate of lime of undoubted purity. ‘To obviate the con- 
tamination arifing from the contact of the porcelain tubes, I de- 
termined to confine the fubject of experiment in fome fubftance 
which had no difpofition to unite with the carbonate. I firft 
tried charcoal, but found it very troublefome, owing to its 
irregular abforption of water and air. 
I THEN turned my thoughts to the conftruGion of tubes or 
cups of platina for that purpofe. Being unable readily to pro- 
cure proper folid veflels of this fubftance, I made ufe of thin 
laminated plates, formed into cups. My firft method was, 
to fold the plate exactly as we do blotting-paper to form a filter’ 
(Fig. 26.); this produced a cup capable of holding the thin- 
neft liquid ; and being covered with a lid, formed of a fimilar 
thin plate, bent at the edges, fo as to overlap confiderably 
(Fig. 28.), the carbonate it contained was fecured on all fides 
aa the contact of the porcelain tube within which it was 
placed. Another conyenient device likewife occurred: I 
wrapt a piece of the plate of platina round a cylinder, fo as 
to form a tube, each end of which was clofed by a cover 
like that juft deferibed (Fig. 27. and 29). (In figure 26. and 
27. thefe cups are reprefented upon a large fcale, and in 28. 
and 29. nearly of their actual fize). This laft conftruction 
had the advantage of containing eight or nine grains of car- 
bonate, whereas the other would only hold about a grain 
and a half. On the other hand, it was not fit to retain a 
thin liquid; but, in moft cafes, that circumftance was of no 
confequence; and I forefaw that the carbonates, could not 
thus 
