2:9?' Dr. Priestley’s Experiments and Obfiervationr 
thing that I know to the contrary, the fame tube may ferve: 
for a very great number of procefies, and perhaps the change 
made- in the in fide Surface may protect it from any farther action 
of the water, if the tube be of Sufficient thickneis but this 
can only be determined by experiment. 
Some eftimate of what may be expected from this method of 
procuring inflammable air may be formed from the following 
©bfervations. About twelve inches in length of a copper tube, 
three-fourths of an inch in diameter, filled with iron turnings 
(which are more convenient for this purpofethan iron filings , as 
they do not lie fo clofe,. but admit the fleam to pafs through 
their interfaces) when it w r as heated, and a Sufficient quantity of 
fleam palled through it, yielded thirty ounce meafures of air in 
fifty feconds ; and eighteen inches of another copper tube, an 
inch and a quarter in diameter, filled, and treated in the fame 
manner, gave two hundred ounce meafures in one minute and 
twenty-five feconds %. fo that this larger tube gave air in pro- 
portion to its folid contents compared with the Smaller;, but to 
what extent this might be depended upon I cannot tell. How- 
ever, as the heat penetrates fo readily to fome diflance, the rate 
of giving air will always be in a greater proportion than that of 
the Ample diameter of the tube. 
The following experiment was made with a view to afcertaiti: 
the quantity of inflammable air that may he procured in this, 
way from any given quantity of iron. Two ounces of iron,., 
or 960 grains, when diflolved in acids, will yield about 800 
ounce meafures of air ; but treated in this manner it yielded 
1054 ounce meafures* and then the iron had gained 329 grains 
in weight, which is little Short of one-third of the weight of 
the irorio. 
Confideringy 
Or 
