rhospludic Mdtcrinls ttscdfor Af/n'mlfurrd Purposes. 309 
quantities of bone or of bone-ash are employed in. testinp: for 
fluorine. It strikes me that such an etching would be readily 
produced if bones really did contain from 3'52 to 3'85 per 
cent, of fluoride of calcium, as calculated by Heintz. More- 
over, such a proportion of fluoride of calcium would admit of at 
least an approximate determination. I have endeavoured to 
determine quantitatively the proportion of fluorine in recent bones 
and in bone-ash ; but although I have carefully tried all known 
methods for determining small quantities of fluorine, I have 
utterly failed in all attempts, and am inclined to think that the 
proportion of fluoride of calcium in recent bones is very much 
smaller than Heintz imagines it to be. To verify this supposi- 
tion, I added 1 per cent, of finely-powdered fluoride of calcium 
to pure phosphate of lime, and then was able to ascertain the 
amount of fluorine with tolerable precision. This is not the 
place to mention the details of my experiments, but the conclu- 
sion to which they seem to lead is that recent bones contain 
only a small amount of fluoride of calcium, and not 3^ per cent, in 
round numbers as stated by Heintz. This much is certain, that 
the mineral portion of bones contains a certain quantity of lime, 
which is neither united with carbonic acid, nor with phosphoric 
acid as tribasic phosphate. 
If it be highly improbable that this excess of lime exists in 
bone-ash entirely in the state of fluoride of calcium, the question 
naturally arises, In what combination does it occur? As lime 
and phosphoric acid unite together in so many different propor- 
tions, and as many of these compounds are basic in their cha- 
racter, it is highly probable that bone-ash contains a more basic 
phosphate of lime than has hitherto been supposed to exist. 
It is a matter of considerable practical importance that the 
composition of the precipitate which is obtained on adding 
caustic ammonia to a dilute solution of bone-ash in hydrochloric 
ncid should be known with certainty. This precipitate, con- 
sisting of phosphate of lime and a little phosphate of magnesia, 
is usually called bone-earth. It was considered by Berzelius to 
be a combination of 8 equivalents of lime and 3 equivalents of 
phosphoric acid ; its formula accordingly is 8 C O + 3 P Oj. 
Other chemists assign to it the formula 3 C O 4- P Oj. Indeed, 
most scientific chemists of the present day have, for good reasons, 
given up the older formula, and consider bone-earth to be prin- 
cipally tribasic phosphate of lime, — that is a combination of 
3 equivalents of lime and 1 equivalent of phosphoric acid. 
In commercial analyses the amount of bone-earth is usually 
determined by precipitation ; but since some chemists very pro- 
perly prefer to ascertain the total amount of phosphoric acid in 
the article submitted to analysis, and to calculate subsequently 
