250 Zinc, as a Covering for Buildings. 
roofs, that if it is suffered to stand for some time exposed to the air, 
the suboxide gradually takes oxygen from the atmosphere, and is 
thus converted into the.insoluble white oxide before mentioned, and 
is then precipitated in the form of a white powder. ‘To test its pu- 
rity by this method, I have kept water from a zinc roof exposed in 
clean glass vessels for several days, without any, the slightest ap- 
pearance of a precipitate, or even a pellicle upon the surface. And 
what is still better as a test, I have kept it for several days in closed 
bottles with oxygen gas, and subjected it to frequent agitation, with- 
out the least appearance of a precipitate, or any diminution of trans- 
parency. I must think, therefore, that if such water contains the 
suboxide of zinc, its presence is not to be detected in this way. 
That the quantity of zine dissolved in water must be exceedingly 
small, is obvious from the following consideration. A sheet not more 
than the fortieth of an inch in thickness, would probably last at least 
half a century, on the roof of a building. Indeed, for any thing we 
know as to the rate of its oxidation, it might last for centuries. The 
concurrent opinion of chemists, and this confirmed by observation 
and experiment, so far as these have extended, is, that after the gray 
oxide is once formed, any further change takes place scarcely at all, 
or with extreme slowness. But on the supposition that it would last 
only fifty years, the whole quantity of rain which falls in the course 
of a year, or about three feet on the level, would dissolve the two 
thousandth part of an inch in thickness of zinc. This, to produce 
any appreciable effect, must be one of the most virulent of poisons, 
equal at least to prussic acid. But so far from being an active poi- 
son, it remains to be shewn that it is poisonous at all, even if a mi- 
nute portion of it did mingle with the water. The white oxide of 
zine is not poisonous, and the inference seems to be gratuitous that 
this is so. 
It is due no less to the public than yourselves, that the truth upon 
this subject should be known and promulgated. I am quite satisfied, 
for one, that we are not in the least danger of being poisoned by the . 
use of water from zinc roofs. The portions of this water which I 
have examined, could not be distinguished from pure river water by 
any test that I have been able to apply to it. I feel myself wat- 
ranted, therefore, in the conclusion, that it ceneg suffered no detertor- 
whatever from the zine. 
3. A third objection is that zine affords inadequate protection 
‘ 
. 
against fire. 
