On progressively trying solutions of weaker and weaker preservative power, it will be remarked, that the quantity of the detached powder, and the proportion of carbonate in it, progressively increase; and likewise, that what is formed on the lead adheres more and more loosely. In distilled water and weak solutions of acetate of soda, or nitrate of potass, the lead never becomes so firmly encrusted, but that gentle agitation of the water will shake off the powder.
It is worthy of notice that, although a small quantity of lead is dissolved by distilled water after it has remained some time in contact with the metal, yet not a trace is found in solution where a protecting salt is present. In solutions even weakly preservative I never could detect any lead dissolved. Thus, in distilled water containing a 4000th of muriate of soda, or a 160th of nitre, the lead lost weight, and loose crystals of carbonate were formed; yet even after thirty days no lead could be found in solution by the process with which I have always detected it in pure distilled water. Free exposure to the air is probably in part the cause of this. For it will be seen afterwards that some natural waters in passing through a long course of lead pipes, within which the action goes on without direct access of the atmosphere, contract an impregnation, which is invisible when the water is newly drawn, but after a few hours’ exposure to the air shows itself in the form of a white film and milkiness.
The general result of these experiments appears to be, that neutral salts in various, and for the most part minute, proportions, retard or prevent the corrosive action of water on lead,—allowing the carbonate to deposit itself slowly, and to adhere with such firmness to the lead as not to be afterwards removable by moderate agitation, adding subsequently to this crust other insoluble salts of lead, the acids of which are derived from the neutral salts in solution,—and thus at length forming a permanent impermeable skreen, through which the action of the water cannot any longer be carried on.
An important subject of inquiry regards the natural causes by which the preservative power of the neutral salts is impaired. This topic I have not hitherto been able to examine with all the care which is desirable.
From the effect of the water of Edinburgh when highly charged with carbonic acid, I was led to infer in former editions of this work that an unusual quantity of carbonic acid is a counteracting agent. For if Edinburgh water charged with it be corked up with some lead rods in a phial half-filled with water, and half with atmospheric air, the lead, which in common Edinburgh water, as will presently be mentioned, hardly loses any of its brilliancy for six or seven days, becomes quite white in twelve or sixteen hours. Subsequent experiments by Captain Yorke seemed to him to render this conclusion doubtful; nor do I attach much consequence to the observation just quoted. On the other hand it is said Professor Daniell has found all waters dissolve lead, if they contain an excess of carbonic acid.[[1242]] The point would be best settled by the effect of a natural carbonated water passing through a long lead pipe.
On the Action of Natural Waters on Lead.
The preceding observations on the action of water on lead may be resorted to for explaining many interesting facts, and correcting some erroneous statements, which have been published by authors as to the corrosion of lead by natural processes.
Rain and Snow-Water.—It has been stated by Dr. Lambe that rain-water does not corrode lead, that “its effect is so slight as not to be discernible within a moderate compass of time.”[[1243]] But this observation is far from being correct. Rain or snow-water, collected in the country at a distance from houses, and before it touches the earth, being nearly as pure as distilled water, ought to act with equal rapidity on lead. I have accordingly found by a comparative experiment with that mentioned in p. [401], that in twelve ounces of snow-water, collected ten miles west from Edinburgh, and at some distance from any house, twelve lead rods weighing 340 grains lost two grains in eight days, and the usual crystals began to form in less than an hour. But when collected in a great city, rain or snow-water is much impaired in activity. Thus in an experiment made with eaves’-droppings collected from the roof of my house in Edinburgh, after half an hour of gentle rain from the south-east,—the first rain which had fallen for several weeks,—there was no action at all. Yet even when collected in a great city, and in circumstances which at first sight would appear not very favourable to its action,—for example from eaves’-droppings a few hours after the beginning of a shower,—it retains a little of its corroding property; and when collected in like manner after twelve or twenty-four hours’ rain, it corrodes almost as rapidly as distilled water. Thus with four ounces of eaves’-droppings collected after the shower last alluded to had continued four hours, the crystalline powder began to cover the bottom of the glass in five hours, and in nine days three lead rods weighing fifty-seven grains lost a fifth of a grain. And in another experiment made with eaves’-droppings after a day’s steady rain from the north-east, the powder began to form in half an hour, and the loss sustained by the lead in thirty-three days was a grain and a third, being very nearly what is lost in distilled water during the same time.
We must obviously be prepared to look for an explanation of these differences in the relative purity of the different waters. Accordingly, in the eaves’-droppings at the beginning of the shower the nitrates of baryta and silver caused, the former a distinct, the latter a faint precipitation, which, as oxalate of ammonia had no effect, arose from the presence of alkaline sulphates and muriates: but after a four hours’ shower nitrate of baryta alone acted, and caused merely a faint haze: and after a twenty-four hours’ shower, as well as in snow-water from the country, none of the three tests had any effect whatever.
Hence, perhaps even in a town, but at all events certainly in the country, it would be wrong to use for culinary purposes rain or snow-water which has run from lead roofs or spouts recently erected. When the roof or spout has been exposed for some time to the weather the danger is of course much lessened, if not entirely removed; because exposure to the weather encrusts it with a firmly adhering coat of carbonate, through which, as already observed, even distilled water will not act. But I believe it would be right to condemn the turning even old leaden roofs to the purpose of collecting water for the kitchen. Although the purest rain-water cannot act on them when it is once fairly at repose, we do not know what may be the effect of the impetus of the falling rain on the crust of carbonate; and if the crust should happen to be thus worn considerably, or detached by more obvious accidents, the corrosion would then go on with rapidity as long as the shower lasted. Acid emanations too disengaged in the neighbourhood, and other more obscure causes may enable rain-water actually to dissolve even the crust of carbonate.