This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
Mr. Fuller has also experimented a little with this article in killing insects upon greenhouse plants, particularly the green fly (Aphis), which, as everybody knows, is a great pest, and one not readily destroyed, except by fumigating with tobacco - not a very agreeable operation to perform upon parlor plants or in a conservatory attached to a dwelling. My first experiment with this soap was a decided success, operating upon two hundred roses just in bloom, and it was conducted as follows: Into a pail of warm water I put a lump of soap the size of a small hen's egg. The soap was cut up into small pieces and the water agitated until it was all dissolved, forming a warm suds. The water should not be too hot, but if not above 120° or thereabout, it will do no harm. Into this suds each rosebush was plunged (holding the pot inverted in the hand and kept there about a half minute). After plunging, the plants were set aside for a few minutes, then dipped in the same way into clean water, shaking them about thoroughly, washing the leaves, and then returned to their former place in the house.
Whether it was the soap or warm water that killed the green fly I will not say, but there is one thing certain, they are all dead.