good plan to partly satisfy this by allowing the molten zine to previously act on a piece of scrap wrought-iron.

Small articles of solid iron or steel are galvanized preferably by the following method. The articles are cleansed in a revolving barrel or tumbling box containing sand, which chafes the iron and removes the scale. A solution is made by saturating with sheet zine 10 parts of hydrochloric acid, and, when the evolution of gas has ceased, dissolving in it 1 part of muriate or sulphate of ammonia. The iron articles are heated and plunged in this solution for an instant, if of the right heat, they will dry at once on removal and be covered with crystals. Prepare a bath of molten zinc as before, removing all oxide and throwing in plenty of salammoniac to stop further oxidation. Heat the articles, dip them while quite dry into the zinc, shake off the superfluous metal, and cool in water. Small articles may be held in a wrought-iron basket when dipping into the zinc.

It may have been noticed that the process of galvanizing proper in both of these two methods is the same, the only difference being in the cleansing processes preceding the galvanizing. Two or three more methods of preparing the iron or steel for the galvanizing bath may now be noted. Immerse the iron articles for a few hours in muriatic acid diluted with twice its weight of water, and then wash thoroughly in hot water and scrub with brush and sand. The final preparation for the bath of molten zine is immersion in a hot solution of 1 pound of salammoniac to 2 gallons of water. Dry before galvanizing. First remove all scale by passing through a bath of 1 part of muriatic acid and 4 parts of water. After brushing and scraping,

pass through a fresh bath of 1 part of muriatic acid, 4 parts of water, and 1 ounce of salammoniac to every gallon of solution, and then dry in a hot oven. Scour with sand all scale and rust from the surface of the metal, and remove all grease and oil by boiling in a solution of caustic soda. Immerse in dilute muriatic acid, scrub with a metallic brush, and rinse in hot water, afterwards drying thoroughly. The molten metal is liable to spit if the article is passed into it wet. It is even possible for slight explosions to occur if moisture is left among the laps and rivets.

Perhaps the most general method of galvanizing sheet steel or iron is the one by which the metal first receives a preparatory coat of tin. The sheet metal is passed through baths of dilute muriatic acid, scoured with sand and otherwise made perfectly clean. A bath is prepared in a wooden vat by adding 1 part of a saturated solution of metallic tin in concentrated muriatic acid to 600 or 800 parts of water. The preparation of the tin solution occupies from two to three days. At the bottom of the vat is a thin layer of finelygranulated zinc, on top of this being a clean iron or steel plate, which in its turn is covered with granu lated zinc, and so on until the bath is full. The zinc, iron, and solution form a weak galvanic battery, tin being deposited from the solution on the iron plates, a coat sufficiently thick for the purpose being obtained in about two hours. The plates are removed, and immediately carried by rollers through a bath of molten zine covered with a thick layer of salammoniac mixed with earthy matter to lessen its volatilization. The speed with which the rollers revolve practically determines the thickness of the zinc coat on the plates.

Owing to the under coating of tin, galvanized plates prepared by this process have a crystalline appearance.

Not only steel and iron, but brass and copper utensils are often galvanized, there are two or three processes by which this can be done, though they are chemical rather than metallurgical. A simple method is to boil the brass or copper in a solution of chloride of zinc, adding at the same time a small quantity of zine turnings to the solution. Another process is to cover granulated or powdered zinc, contained in a wooden vessel, with a concentrated solution of salammoniac. Heat to about the boiling point, and immerse the copper or brass articles, which should be chemically clean. A firm coating of zinc will be deposited in a few minutes.

The more important seams or joints used in sheetmetal work are illustrated in Fig. 146.

Lap Seam. No. 1 shows how sheet-metal plates are arranged for a lap seam to be soldered.

Countersunk Lap Seam. This is shown in No. 2, it will be noticed that the edge of one of the plates is bent down, so that the edge of the other plate to be joined to it may lie in the shoulder formed by the part bent down.

Folded Seam. The method of preparing the edges of plates for a folded seam are shown in No. 3.

Grooved Seam. This is shown in No. 4. It may be seen that it is practically the same as No. 3, only one plate is countersunk.

Countersunk Grooved Seams. The seam shown in No. 5 is used when an unbroken surface is required on the outside of an article.

Double Folded Seam. This is shown in No. 6. and is used with thick plates, where these when joined are required to present an unbroken surface.

Zinc Roofing Joint. The joint shown in No. 7 admits of the expansion and contraction of the zinc sheets.

Rivetted Lap Seam. No. 8 shows a rivetted lap seam. The amount of lap should not be less than three times the diameter of the rivet.

Brazing Joints. A brazing joint for thin metal is shown in No. 9, the edge of plate A is cut to form laps and these laps are arranged alternately over and under the edge of plate B. A brazing joint for thick metal is shown in No. 10. It is practically the same thing as a carpenter's dovetail joint.

Circular Lap Seam. No. 11 shows how the edge of the bottom of a can is bent up previous to soldering.