Sainte-Croix thus wrote to Belleguise:

"DEAR FRIEND,--Is it possible that you need any more talking to about the
matter you know of, so important as it is, and, maybe, able to give us
peace and quiet for the rest of our days! I really think the devil must
be in it, or else you simply will not be sensible: do show your common
sense, my good man, and look at it from all points of view; take it at
its very worst, and you still ought to feel bound to serve me, seeing how
I have made everything all right for you: all our interests are together
in this matter. Do help me, I beg of you; you may feel sure I shall be
deeply grateful, and you will never before have acted so agreeably both
for me and for yourself. You know quite enough about it, for I have not
spoken so openly even to my own brother as I have to you. If you can
come this afternoon, I shall be either at the house or quite near at
hand, you know where I mean, or I will expect you tomorrow morning, or I
will come and find you, according to what you reply.--Always yours with
all my heart."

The house meant by Sainte-Croix was in the rue des Bernardins, and the
place near at hand where he was to wait for Belleguise was the room he
leased from the widow Brunet, in the blind alley out of the Place
Maubert. It was in this room and at the apothecary Glazer's that
Sainte-Croix made his experiments; but in accordance with poetical
justice, the manipulation of the poisons proved fatal to the workers
themselves. The apothecary fell ill and died; Martin was attacked by
fearful sickness, which brought, him to death's door. Sainte-Croix was
unwell, and could not even go out, though he did not know what was the
matter. He had a furnace brought round to his house from Glazer's, and
ill as he was, went on with the experiments. Sainte-Croix was then