When the intern was carrying in the aunt, the attending nurse mentioned having to administer a bubble bath. We all know they are relaxing, but I never figured they were or possibly still are therapeutic. BTW, does any one know when bubble baths became popular? I am assuming in the early 20th century, but it might have been earlier.
The late 1930s. The light duty detergent powder Swerl was advertised for both cleaning (dishes and hand laundry) and bubble baths. Before that there were no chemicals that were adequate to make a bath lastingly foamy. It was possible to do it with soap flakes or powder, but only by making the water as soapy as dish or laundry water; saponins could be used in a mechanically aerated bath. But Swerl and Dreft were able to make a light fluff atop a bathtub of water at lower concentrations than would be needed to make the bath an effective detergent solution. Also they were preceded by the type of bubble bath that fizzes but doesn't foam: bath fizzies.
By 1950 foam baths had become something of a cliche and a joke. I doubt they were ever seriously used for mental health. Conceivably the line in the movie meant the type of bubble bath that doesn't produce foam but is mechanically aerated, such as Jacuzzi, which was devised for physical therapy. Fizzies had also been promoted informally for putatively therapeutic uses if they contained stimulating oils such as of pine -- but so were non-fizzing, non-foaming bath salts and oils. If that line was in the original 1944 play, foaming baths might still have been enough of a novelty that audiences might've believed they could be tried therapeutically for mental health.
For psychoses more extreme COLD baths were tried for a while. During this period were built mental health facilities that had a room with multiple bathtubs for that purpose. People looking at those facilities years later when the rooms were disused may have wondered, how dirty were the patients that they needed baths en masse?
In the 1940s and 1950s bubble bath became a way for women to appear to be bathing naked on film or stage, the foam covering their breasts or actually swimsuits. Preparations for foaming bath water came to be fragrance products sold by the Avon lady and occasionally as gifts in drug stores.
It wasn't until circa 1960 that non-fragranced bath foams like Bub, Bubble Club, Matey, and Mr. Bubble powders, and Soaky liquid, became grocery products promoted for routine use as "family" products alongside soap in children's baths. Their formulas were typically modeled on the high suds all-purpose household detergents of the time. These bubble baths were heavily advertised for a decade, by which time they'd become habitual and didn't need to be advertised much any more. During that time (and before they were weakened in 1970 due to complaints of urinary and skin irritation) some of them claimed to clean skin without soap or rubbing, but were used at too high a dilution for that to work, I think; but they do prevent soap from leaving a bathtub ring in hard water.
Here's a piece about how the movie introduced to one viewer the subject of psychiatric hydrotherapy:
https://brightlightsfilm.com/dream-behind-reality-amiable-lunacy-blithe-brutality-harvey/