A quote from the caption at the bottom of your post: "The book, a highlight of the Loewentheil Family Collection, is one of the finest surviving Civil War photograph albums, with 265 rare photographs by the preeminent photographer Mathew Brady and others. It contains unusual images of infantry, artillery, and cavalry units preparing for battle, military field operations, gun emplacements and camp scenes as well as rare portraits of Union and Confederate officers, prisoners of war and Abraham and Mary Todd Lincoln."

This would seem to indicate that this is most likely a Union hospital. What is it that makes you think it is Confederate?