(Jo clears her throat) and says 'THANK YOU' for caring enough to wonder about this.
And I haz reasons (excuses):
As to the word 'antiseptic'. Millennia ago, long before they understood the mechanism by which it worked, folks knew some stuff discouraged infection. By mid-C18 folks had a word for that and it's the same one we use today. Antiseptic. The OED gives us a 1751 "Myrrh in a watery menstruum was 12 times more antiseptic than salt water."
The writer's problem is that 'antiseptic' sounds very modern.
So I sat for a while pondering whether I should use it. This is classic historical writer dilemma and one of the things that drives sensitive souls to drink -- coffee if not brandy, anyway. 'Historical Word Problem' hits me three, four times a book.
But antiseptic is a couple generations prior to date-of-story, so I went ahead and laid it down.
I also considered the vexed question of which medications would be carried as mix-it-yourself powders and which would be aqueous solutions or tincture-of-this-and-that. It seemed to me the choice depended on the exactness with which the solution must be prepared and how quickly the solution would be needed when called for.
I posited that Justine's mixture takes a good long while to go into solution -- thus the boiling water -- and is likely to be needed PDQ, if needed at all. Bottled at the source, it can be used immediately. Prepared In the field, it would need hot water and a long while to dissolve. This is the same reason a modern first aid kit intended for use in the outback would have its antiseptic in liquid, ready to use, form.
Justine includes one bottle for immediate use and also the powder for mixing a further supply.
I have her dissolve in water rather than alcohol because this is a water-soluble powder. Oddly, I find no indication folks thought of alcohol as antiseptic in 1800. It may just be I haven't researched it enough.
Carrying liquids could feel a little 'off' to the reader for several reasons. I think it's because liquid is heavy. Some part of our mind is reluctance to see clunky bottles of liquid cumbering up a medical kit that has to be carried through the jungles of Borneo or wherever.
The medical cases of the era were heavy.