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 zinc dissolved in water mwsi 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 

 zinc 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 war- 

 ranted, therefore, in the conclusion, that it has suffered no deterior- 

 ation whatever from the zinc. 



3. A third objection is that zinc affords inadequate protection 

 against fire. 



