Although I have not been troubled with gas tar on my pipes at any time, yet, from my knowledge of gas tar in many experiments, I know the cure. If a thick coat of whitewash of lime be put on, after scraping off with a knife all it is possible to, then the lime absorbs all the rest, and if one coat will not do it two or three surely will. It may be found necessary to rub off each coat if much blackened. The lime has a great affinity for the tar and will absorb it all out, unless it (the tar) is put on the inside; then this plan will not do. There is no need whatever, in my opinion, to take down the pipes if the gas tar is only on the outside.