Page:The American Cyclopædia (1879) Volume X.djvu/254

 24:8 LEAD ium nor convulsions being present. The mor- tality in encephalopathy is very high, more than one half of all the cases proving fatal. Of more importance than the treatment of lead disease is the adoption of means for its prevention, and in this regard there is often much to contend with in the indifference of operatives to sanitary measures. Thorough ventilation and scrupulous cleanliness would in most instances prevent the contraction of the disease. Prophylactic remedies have been much used, those proving the most useful being drinks acidulated with sulphuric acid and milk. Cases of poisoning of animals in the vicinity of lead works are not uncommon. This subject was investigated by Dr. George Wilson in 1852, and the results were communicated to the royal society. He found the herbage in the neigh- borhood of a lead-smelting establishment to be impregnated with carbonate of lead, as well as the water in a stream used for washing ore. Fourteen horses and some cows died, it was believed, from the effects of the lead. Exami- nation of the tissues of the animals showed the presence of lead in two cases, but not in the others. The lead found was largely pres- ent in the spleen. Instances have been record- ed of the death of cows from swallowing the " bullet spray " scattered in target practice. The frequent occurrence of cases of poisoning resulting from the very general use of lead pipe in the conveyance of water into dwellings, has led to investigations by many chemists into the action which different waters exert on this metal. Bright lead remains unchanged in per- fectly dry air or in pure water deprived of air and protected from contact with it ; but in a moist atmosphere, or in rain water, its bril- liancy is soon dulled, and its surface is covered with a thin film of oxide, which adheres closely to the metal and protects it from further oxi- dation. The oxide, however, is partially solu- ble in water, and is no sooner taken up by this than it combines with any carbonic acid gas present or absorbed from the air, forming with it a film made up of silky scales of hydrated oxycarbonate of lead. More lead is then oxi- dized, dissolved, and converted into carbonate, and so the process of corrosion goes on. The oxycarbonate is almost insoluble in pure water, this taking up of it only about -fa of a grain to the gallon ; and so perfectly does this sepa- rate from water, that if distilled water hold- ing four or five grains of oxide of lead to the gallon be exposed to the air, the carbonic acid soon imbibed will cause the precipitation of silky crystals of the hydrated oxycarbonate, leaving in solution not more than one part of the metal to 4,000,000 of the liquid, or -^ of a grain to the gallon. But an excess of carbonic acid gives to the water the property of dissolv- ing this carbonate of lead in the same way that it acquires also the property of dissolv- ing carbonate of lime or limestone. So far it seems therefore that carbonate of lead is as likely to be found dissolved in water that comes in contact with the metal, as carbonate of lime in water flowing over limestone. But the presence of certain salts in the water, even in very minute quantity, modifies materially this action. The sulphates, phosphates, and car- bonates of the alkalies, and the sulphates, car- bonates, &c., of the alkaline earths, or indeed any neutral salt, the acid of which can pro- duce with lead or its oxide an insoluble com- pound, greatly diminish this action, even if present in the water to the amount of only four or five grains in the gallon. Bicarbonate of lime, which is almost always present in spring water, is especially remarkable for its protecting influence. The action of these salts is to form insoluble precipitates, which accu- mulate upon the surface of the lead, and cover it with a protecting lining. The protection is not, however, uniformly efficient, for there are certain other salts and acids which exert a contrary influence, and frequently completely overpower the beneficial effects of the first class named. Such are the chlorides and ni- trates, and especially nitric and nitrous acids, in solution. These acids are generated in all waters containing decomposing animal matter, and therefore must be almost universally pres- ent to some extent. Dr. Medlock, who has given much attention to this subject, goes be- yond other chemists in the importance he as- cribes to the influence of these acids. The action of any water on lead, he states, is en- tirely due to the presence of nitrous and nitric acid, resulting primarily from the decomposi- tion of organic matters and of ammonia con- tained in the water ; and further, that water deprived of these acids, and of substances ca- pable of producing them, has no action on lead, and may be conveyed with perfect safety through leaden pipes or stored in leaden cis- terns. He devised a method of removing the nitrates and also organic matters from water, unless the latter be present in great excess. It was by suspending coils of iron wire or pieces of sheet iron in the water, and after a time filtering off the deposit. The iron decom- poses the nitric acid, being itself peroxidized, and nitrous gas is liberated, which oxidizes the carbonaceous matters, so that they are re- solved into carbonic acid and a lower oxide of nitrogen. The original paper of Henry Medlock, Esq., " On the Reciprocal Action of Metals and the Constituents of Well and River Waters," is the 24th article of vol. xiv. (4th series) of the "London, Edinburgh, and Dub- lin Philosophical Magazine " (1857). The quan- tity of lead in solution in the gallon of water which will suffice to produce injurious effects, depends very much on the individual constitu- tion and on the length of time that the water continues to be used. Dr. Penny, professor of chemistry at Glasgow, cites an instance of the health of a whole community being deranged by water containing only one ninth of a grain of lead to the gallon ; and also quotes the con- clusion of Dr. John Smith of Aberdeen, that