Before proceeding to the detection of lead in complex organic mixtures, some remarks will be required on its relations to medical police. Here the various ways in which it is apt to be insidiously introduced into the body, chiefly by the action of chemical agents on metallic lead itself, will come under consideration.
Of the Action of Air and Pure Water on Lead.
When lead is exposed to the air it becomes tarnished. This arises from a thin crust of carbonate of lead being formed; for the crust dissolves with brisk effervescence in acetic acid. The formation of carbonate is accelerated by moisture and probably by the presence of an unusual proportion of carbonic acid in the air.
The action of water on lead, which is of much greater consequence, has been made the subject of observation by the curious for many ages. The Roman architect, Vitruvius, who, it is believed, nourished in the time of Cæsar and Augustus, forbids the use of this metal for conducting water, because cerusse, he says, is formed on it, which is hurtful to the human body.[[1224]] Galen also condemns the use of lead pipes, because he was aware, that water transmitted through them contracted a muddiness from the lead, and those who drank such water were subject to dysentery.[[1225]] If we trace the sciences of architecture, chemistry, and medicine downwards from these periods, nothing more will be found than a repetition of the statements of Vitruvius and Galen, with but a few particular facts in support of them, till we arrive at the close of the last and beginning of the present century.
The first person that examined the subject minutely, was Dr. Lambe of Warwick; who inferred from his researches, that most, if not all, spring waters possess the power of corroding and dissolving lead to such an extent as to be rendered unfit for the use of man, and that this solvent power is imparted to them by some of their saline ingredients.[[1226]] The inquiry was afterwards undertaken more scientifically by Guyton-Morveau; who, in opposition to Dr. Lambe, arrived at the conclusion, that distilled water, the purest of all waters, acts rapidly on lead by converting it into a hydrated oxide, and that some natural waters, which hardly attack lead at all, are prevented doing so by the salts they hold in solution.[[1227]] A few years later Dr. Thomson of Glasgow also examined the subject, and, assenting to Dr. Lambe’s proposition, that most spring waters attack lead, maintains nevertheless that the lead is only held in suspension, not in solution; and that the quantity suspended in such waters, after they have passed through lead pipes, pumps, and cisterns, is too minute to prove injurious to those who make habitual use of them.[[1228]] In the first edition of this work an extended account was given of an investigation I made into the whole subject of the action of different waters on lead.[[1229]] Additional observations were afterwards published on the same point by Captain Yorke,[[1230]] and by Mr. Taylor.[[1231]] And I have added some new facts in a late paper.[[1232]]
The inquiry is of so great practical consequence, that I need not offer any apology for reproducing it here in detail, with such additions as ulterior experience and the researches of others enable me to make. Professor Orfila takes no notice of this important subject, except in a few lines containing several inaccurate statements.[[1233]]
Distilled water, deprived of its gases by ebullition, and excluded from contact with the air, has no action whatever on lead. If the water contains the customary gases in solution, the surface of the metal, freshly polished, becomes quickly dull and white. But if the surface of the water be not at the same time exposed to the air, the action soon comes to a close.—When the air, on the other hand, is allowed free access to the water, a white powder appears in a few minutes on and around the lead; and this goes on increasing till in the course of a few days there is formed a large quantity of white matter which partly floats in the water or adheres to the lead, but is chiefly deposited on the bottom of the vessel. If this experiment be made with atmospheric air deprived of carbonic acid, the white substance puts on the form of a fine powder, which I find to be a hydrated oxide; for when dried at 180°F. it gives off water on being heated to redness, and dissolves without effervescence in weak nitric acid.—But if the surface of the water be exposed to the open air, the substance formed consists of minute brilliant pearly scales, which with the aid of a powerful microscope are seen to be thin equilateral triangular tables, often grouped into hexaedral tables, or worn at the edges into the form of rosettes. This substance, which has a pale grayish hue when dried, I have ascertained to be a carbonate of lead, consisting of two equivalents of neutral carbonate and one of hydrated protoxide.[[1234]] The formation of carbonate takes place with considerable rapidity. In twelve ounces of distilled water, contained in a shallow glass basin loosely covered to exclude the dust, twelve brightly polished lead rods weighing 340 grains, will lose two grains and a half in eight days; and the lead will then show evident marks of corrosion. The process of corrosion goes on so long as atmospheric air is allowed to play freely on the surface of the water. In twenty months I have obtained 120 grains from an ounce of lead rods kept in 24 ounces of distilled water.
During these changes, a minute quantity of lead is dissolved. This is best proved by carefully filtering the water, then acidulating with a drop or two of nitric acid, and evaporating to dryness. I have never failed to detect lead in the residue by expelling the excess of nitric acid by heat, dissolving it in distilled water, and applying hydrosulphuric acid, hydriodate of potass, and chromate of potass to the solution. The lead is first dissolved in the form of hydrated oxide. For, if the air admitted to the water be deprived of carbonic acid, a clear liquid is obtained by filtration, and this is turned brown by hydrosulphuric acid. But a great part of the hydrate is speedily separated in the form of carbonate. For the filtered liquid speedily becomes turbid if exposed to the air; and on evaporating it, the residuum dissolves in weak nitric acid with brisk effervescence. Captain Yorke estimates the quantity dissolved when the water is saturated at a 10,000th part.[[1235]]
By far the greatest part of the lead, however, which disappears, will be found in the white pearly crystals. This crystalline powder is not,—as alleged by Guyton-Morveau, and after him by some systematic writers, a hydrated oxide of lead, but, as stated above, a particular variety of carbonate, containing more hydrated oxide than exists in common white lead. At first I thought it was neutral carbonate. Captain Yorke was led to suppose it hydrated oxide. In 1842 I found that, if it be exposed for some time to the action of aërated water after the lead has been removed, it invariably consists of two equivalents of neutral carbonate and one of hydrated oxide.
It will be inferred from the preceding facts, that distilled water for economical use should never be preserved in leaden vessels or otherwise in contact with lead. Even the distilled water of aromatic plants should not be so preserved, because the essential oil which communicates to them their fragrance does not take away the power which pure distilled water possesses of acting on lead. This fact was first announced in the second edition of the present work. A druggist in Edinburgh requested me to examine a reddish-gray crystalline, pearly sediment formed copiously in a sample of orange-flower water. I found this to be carbonate of lead coloured by the colouring matter of the water, and obviously produced by the action of the water on lead solder used instead of tin solder, and coarsely and liberally applied to the seams of the copper vessel in which the water had been imported from France. The filtered fluid did not contain a particle of lead. The same observation has been since made by a French pharmaceutic chemist, M. Barateau, who seems at a loss, however, to account for the formation of the carbonate of lead.[[1236]] It appears from an inquiry of MM. Labarraque and Pelletier, conducted at the request of the Prefecture of Paris, that the orange-flower water, which is extensively used there, is often adulterated with lead in solution. They impute this to careless distillation; for then some of the decoction is driven over with the distilled liquid, and consequently produces a fluid which becomes acetous by keeping and dissolves the lead solder of the estagnons or copper vessels. Pure orange-flower water does not acidify by keeping.[[1237]] M. Chevallier in a more recent investigation arrived at the same results, and found that few specimens of the orange-flower water of Paris were altogether free of lead.[[1238]] In none of these inquiries have the authors adverted to the action of pure water in forming carbonate of lead.