Payen’s ozonometer, which is an improvement on the above, is made of red litmus paper with half its surface impregnated with a 1% solution of potassium iodide. The portion of the paper becomes blue by contact with air containing ozone, in consequence of oxidation and the formation of potash. The unimpregnated portion of the paper undergoes no change unless the air contains ammoniacal vapours, and then the paper becomes blue over its entire surface.
M. Davy states that he has obtained very satisfactory results in the estimation of ozone in the atmosphere, by employing a mixture of iodide of potassium and arsenite of potassium.
The value of the ozonometer as an indicator of atmospheric ozone must be looked upon as uncertain, when it is borne in mind that there are other bodies besides ozone frequently present in the air, such as nitrous acid, chlorine, &c., which give similar reactions with the above reagents.
PACK′FONG. Syn. Pakfong, Chinese white copper. An alloy formed by fusing together, in a covered crucible, arsenic, 2 parts, and copper clippings, 41⁄2 to 5 parts, arranged in alternate layers, and covered with a capping of common salt. The product contains about 10% of arsenic.
Prop., &c. White, slightly ductile, and permanent at ordinary temperatures; at a temperature below that of redness it suffers decomposition, with the extrication of fumes of arsenious acid. Formerly much used for the scales of thermometers and other instruments, dial plates, candlesticks, &c. It is now almost superseded by the alloy of nickel and copper called German silver, to which the name is also applied by some recent writers.
PACK′ING. As there is considerable art in packing brittle hollow-ware, as glass, china, &c., in such a way that it will stand exposure to the jolting, blows, and agitation of land carriage, it is better, when it is of much value, or in quantity, to employ a person qualified for the job. A man accustomed to packing such articles may be readily procured at any glassworks or china warehouse for a trifling consideration. When this cannot be done, it must be recollected that the great secret of safe packing consists in the articles being carefully preserved from undue pressure or contact with each other, yet so firmly arranged, and so surrounded with some material as hay, straw, sawdust, &c., that they cannot be shaken into such a condition by the ordinary contingencies of transport. Loose packing must always be avoided.
PAD′DING. Among calico printers this term is applied to the operation of impregnating the pores of their cloth with a mordant. It is now almost exclusively performed by
means of a simple piece of machinery (padding machine), which essentially consists of—a ‘large reel,’ around which the unprepared cloth is wound—a ‘guide roller,’ over which it passes to smooth and adjust it before entering the liquor—a copper cylinder, or ‘dip-roller,’ nearly at the bottom of the ‘mordant-trough,’ under which it is carried from the guide-roller—a half-round polished ‘stretched-bar,’ to give it equal tension—a pair of ‘padded cylinders,’ to remove superfluous moisture—and, lastly, a ‘reel’ to receive the mordanted (‘padded’) cloth. The degree of tension is regulated by a weight suspended on a lever, and motion is given to the whole by an endless band from the driving shaft. This machine is also applicable to many of the operations of dyeing, bleaching, and starching textile fabrics.
PAINT′ER’S CREAM. Prep. Take of pale nut oil, 6 oz.; mastic, 1 oz.; dissolve, add of sugar of lead, 1⁄4 oz., previously ground in the least possible quantity of oil; then further add of water, q. s., gradually, until it acquires the consistence of cream, working it well all the time. Used by painters to cover their work when they are obliged to leave it for some time. It may be washed off with a sponge and water.
PAINT′ING. The art or employment of laying on colour. In the fine arts, the production of a picture or a resemblance in colours on a flat surface. The artistic and mechanical consideration of this subject does not come within the province of our volume; but notices of the leading materials employed by both artists and house painters are given under the respective names. See the various pigments, Colours, Oils, Varnishes, &c., and below.