This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This excellent preparation of arsenic is made, according to the directions of the U. S. Pharmacopoeia, by boiling sixty-four grains of arsenious acid, and the same quantity of pure carbonate of potassa with twelve fluidounces of water, till the acid is dissolved, then adding half a fluidounce of compound spirit of lavender, and afterwards sufficient distilled water to make exactly a pint of the solution. The arsenious acid unites with the potassa of the carbonate, the carbonic acid of which escapes, so that the resulting liquid is simply a solution of arsenite of potassa, slightly coloured and flavoured with compound spirit of lavender. This is important; as the pure solution, without some flavouring addition, might be readily mistaken for water, with very serious consequences. The preparation was introduced into use by Dr. Fowler, who contrived it as a substitute for a patented medicine, which had been previously in use under the name of the tasteless ague drop.
The solution is transparent and of a light yellowish-brown colour, imparted to it by the compound spirit of lavender, which gives it also a slight odour and taste. It is decomposed by substances incompatible with solution of arsenious acid. It is the preparation now almost universally preferred for obtaining the systemic effects of arsenic; and, in my own practice, I have found occasion for no other. It contains half a grain of arsenious acid in each fluidrachm; and ten minims, or about the same number of drops, are the medium dose, to be taken twice or three times daily. The dose may often be lessened to three drops, or increased to fifteen or twenty with advantage. The same rule as to the suspension or diminution of the dose is applicable to this preparation as to arsenious acid; but there is less danger of accumulation than with the latter. The dose for children should be proportioned to the age. (See vol. i. pp. 33-4.)
Fowler's solution has been given by inhalation, in the form of spray, in spasmodic asthma; from one to twenty minims being employed for the purpose in a fluidounce of water.