Creosote is a product obtained in distilling tar. It is an oily, dark liquid, varying in composition according to the quality of the coal from which it is obtained, and containing hydrocarbons of different degrees of volatility and varying antiseptic qualities. Until lately the portions of low specific gravity were considered the best, but experience shows that the lighter portions are volatile and soluble in water, so that the valuable acids may be washed out; a heavy oil, well heated, and with high pressure, gives a better result. The naphthaline is dissolved by the heat, and afterwards fills the pores of the wood and then solidifies.1
"The minute glistening scales generally observable on newly creosoted wood consist of naphthaline, a substance that possesses considerable antiseptic properties; when this substance exists in the liquor in moderate quantities it thickens and confirms its consistency, but when there is a very large proportion ... it makes the liquor too solid." 2
Dr. Tidy's specification for creosote is here summarised.2
1. To be quite liquid at 100° without deposit until the temperature falls to 95°.
2. One-fourth not to distil over in a retort at less temperature than 600°, and this fourth to be heavier than water.
3. To contain 8 per cent of tar acids by analysis with caustic soda and sulphuric acid.
4. No bone oil or shale oil or any oil not distilled from coal tar.
There are two classes of creosoting oils, known in the trade as London oils and country oils.
"The London oils, which consist of those obtained from the gas tar derived from Newcastle coal, contain a large proportion of naphthaline, and are heavier and thicker than the country oils of the Midland districts, which yield a large proportion of tar acids, as they are called."
Previous to 1863 but little of this thin country oil was used, but since that they became more in demand, under the impression that the tar acids were the most valuable part of the oil. Subsequent experiments have shown, however, that the "so called green oils distilling over at a high temperature formed the best portion of the creosoting liquor, and that the importance of the tar acids had been much overrated."
The specific gravity of creosote depends upon the locality in which it is distilled. The material is sold in casks containing from 36 to 38 gallons each.
1 Dent's Cantor Lectures.
2 R.E. Journal.
Hygeian Eoek Building Composition is a bituminous substance used for keeping damp out of houses.
The walls are built in two thicknesses, with a space of about 1/2 inch or more between them, into which as the wall is carried up the composition is run in a liquid state. Existing walls are made damp-proof by adding a lining of tiles or bricks with the composition between. The material is said not only to keep out damp and vermin, but to add to the strength of the wall. It is sold in bags of 1 cwt., which will cover about 21/2 square yards 1/2 inch thick."1