The prestige class necrocarnate at level 1 gains the supernatural ability harvest souls that, in part, says
[Y]ou can perform a short ritual to capture the soul energy of a newly dead corpse. This ritual requires 1 minute of uninterrupted concentration plus the corpse of a living creature that has been dead for no longer than 1 hour per necrocarnate level you possess. At the end of the ritual, you gain a number of essentia points equal to one-half your necrocarnate level (rounded up). This benefit lasts for 24 hours. No corpse can be used for this purpose more than once. (Magic of Incarnum 132)
Is the essentia gained from the supernatural ability harvest souls cumulative? For example, does a level 1 necrocarnate that uses the ability harvest souls twice on two recently dead corpses in as many minutes gain 2 essentia, 1 of which, after 1 day, will be gone 1 min. later than other?
Or, instead,—much like temporary hp from the same source,—does the next application of the supernatural ability harvest souls in the same 24-hour period replace the previous application of the ability harvest souls, updating to their new values the amount of harvested essentia the necrocarnate gains and the duration the necrocarnate's harvested essentia perists? For example, does a level 1 necrocarnate that uses the ability harvest souls twice on two recently dead corpses in as many minutes gain but 1 essentia, the second corpse's essentia replacing the first corpse's essentia and the second corpse's essentia fading 1 day after its acquisition?
Or is there another different way to read this special ability that I'm overlooking?
Note: While Jalas remains 4 levels away from his first level of necrocarnate, his player is considering next level an improved familiar that will leave Jalas unable to enter the prestige class necrocarnate when Jalas is level 8. The DM already knows that, yes, if the effects of the ability harvest souls is not cumulative only one level in necrocarnate for the special ability harvest souls isn't worth it for a meldshaper, but the ability harvest souls becomes staggeringly good if its effects are cumulative… and even worth delaying the acquisition of a glacier snake improved familiar despite the glacier snake's awesome Serpent Kingdoms description: "A glacier snake’s 12-foot body is covered with a thick, brown pelt[;] razor-sharp teeth surround its mouth in a ring, and it has a bony stinger at the tip of its tail" (84). Yes, Jalas's choice is between harvesting the souls of the dead sooner and having a 12-ft.-long furry snake later or vice versa. And, yes, Jalas does look like he's stepped out of an 80s heavy metal album cover.