ert the latter into the former, all we have to do is
to _take away two atoms of lime_.

"Sulphuric acid has a stronger affinity for lime than phosphoric acid.
And when you mix enough sulphuric acid with finely ground three-lime
phosphate, to take away two atoms of lime, you get the phosphoric acid
united with one atom of lime and two atoms of water."

"And what," asked the Deacon, "becomes of the two atoms of lime?"

"They unite with the sulphuric acid," said the Doctor, "and form
plaster, gypsum, or sulphate of lime."

"The molecular weight of water," continued the Doctor, "is 18; of lime,
56; of sulphuric acid, 80; of phosphoric acid, 142.

"An average sample of commercial bone dust," continued the Doctor,
"contains about 50 per cent of phosphate of lime. If we take 620 lbs. of
finely-ground bone-dust, containing 310 lbs. of three-lime phosphate,
and mix with it 160 lbs. of sulphuric acid (say 240 lbs. common oil of
vitriol, sp. gr. 1.7), the sulphuric acid will unite with 112 lbs. of
lime, and leave the 142 lbs. of phosphoric acid united with the
remaining 56 lbs. of lime."

"And that will give you," said the Deacon, "780 lbs. of 'dissolved
bones,' or superphosphate of lime."

"It will give you more than that," said the Doctor, "because, as I said
before, the two atoms of lime (112 lbs.) are replaced by two atoms (36
lbs.) of water. And, furthermore, the two atoms of sulphate of lime
produced, contained two atoms (36 lbs.) of water. The mixture,
therefore, contains, even when perfectly dry, 72 lbs. of water."

"Where does this water come from?" asked the Deacon.

"When I was at Rothamsted," said I, "the superphosphate which Mr. Lawes
used in his experiments was made on the farm from animal charcoal, or
burnt bones, ground as fine as possible--the finer the better. We took
40 lbs. of the meal, and mixed it with 20 lbs. of water, and then poured
on 30 lbs. of common sulphuric acid (sp.g. 1.7), and stirred it up
rapidly and thoroughly, and then threw it out of the vessel into a heap,
on the earth-floor in the barn. Then mixed another portion, and so on,
until we had the desired quantity, say two or three tons. The last year
I was at Rothamsted, we mixed 40 lbs. bone-meal, 30 lbs. water, and 30
lbs. acid; and we thought the additional water enabled us to mix the
acid and meal together easier and better."

"Dr. Habirshaw tells me," said the Doctor, "that in making the
'Rectified Peruvian Guano' no water is nece