This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Your Pelargonium - leaf was forwarded to that well-known authority "M. J. B.," who has expressed the opinion that it has the appearance of being a case of common spotting, and one likely to have arisen from the soil having been allowed to become too dry at some time or other, by which many of the fine rootlets decayed, and in all probability were afterwards gorged with water; and this, added to an imperfect drainage, has no doubt produced the decayed appearance of the leaf sent. Unfortunately the initials of our correspondent were lost, but there is no doubt the reply will be recognised.
In answer to correspondent A. M. A., whether the gas-tar he had plastered over his frame will kill the plants, I once had something similar: I got a pit built for the protection of bedding-plants in winter and for growing Cucumbers in summer. The walls and flues were built with gas-lime, which had a very strong smell of gas. I was afraid of getting my plants destroyed, but, being forced by hard weather, and having no other place for them, they were put in that day it was finished. I was very careful in giving all the air that the weather would allow, and I am happy to say that I did not lose a single plant; indeed they grew strong and healthy after they were taken out. I planted Cucumbers, and had an abundant crop. It has been used as above for three years, and I never saw any ill effect it had upon the plants; but if A. M. A. still fears, let the tar be washed over with hot lime, and that will remove the smell to a great extent.