The association of Soldier's Joy with opiates is offered in several texts that I've seen, but morphine is a fairly "modern" opium derivative that, so far as seen, wasn't much used until early 20th century.

The more common opiate to which the time of the song's apparent origins overlaps would likely have been called laudanum.

Paracelsus, born Phillippus Aureolus Theophrastus Bombastus von Hohenheim (1493-1541) in Salzburg, Austria, a 16th-century Swiss-German alchemist, discovered that the alkaloids in opium are far more soluble in alcohol than water. Having experimented with various opium concoctions, Paracelsus came across a specific tincture of opium that was of considerable use in reducing pain. He called this preparation laudanum, derived from the Latin verb laudare, to praise.[2] Initially, the term "laudanum" referred to any combination of opium and alcohol.

Laudanum was widely available during just about any recent war, and was a main ingredient in many of the "snake oil" patent medicines touted in the late 19th century (US Civil War time), and would have been well known and probably widely used. It's refered to frequently in "cowboy movies" - mostly set shortly after that war - when the old frontier doc treats almost anything.

If the song does refer to an opiate, it most likely would have been called laudanum, although the precise formulation would be no more specifically known than for most modern herbalist concoctions.

Although morphine was known ca. 1804(?), no widespread use was made of it until the invention of the hypodermic needle ca. 1857. It may have been used enough in the Civil War to cause the association to have attached then, but it's unlikely that the opiate association, if it existed earlier, meant morphine.

But back to the song now.

John