Hi Denn and Zebbie,
OK, if the picture doesn't change when a black card is put behind it then I think it must be a dry plate as Denn suggested...that is unless the plate is painted black on the back (instead of having a card) but then you would have said you have a positive not a negative.
However, that presents a problem...gelatin dry plates were only introduced in the 1870s, and these ladies are certainly much earlier than that.
Zebbie - I hate to be a pain, but do you reckon you could take a photo of the whole plate? A pic of it full on, and one taken at an angle to catch the light, would be just great and would really help me to work out what it is. They don't have to be big photos, just small enough to be posted on here.
Prue