IN ROCKS OF IGNEOUS ORIGIN. 
55 
Other substances were then tried with very decisive results. The method of pro- 
ceeding 1 adopted was very much the same as that already described. The minerals 
were very finely powdered in the porphyry mortar and boiled, as before, with dilute 
hydrochloric acid. All were much more readily attacked than the porcelain -clay, 
and yielded solutions containing- a large quantity of alumina and oxide of iron. The 
liquid was separated from the insoluble part by decantation, evaporated nearly to 
dryness, water added, and then an excess of ammonia. The copious, bulky precipi- 
tates obtained were washed and digested in dilute acetic acid, which has the property 
of dissolving with great facility both oxide of iron and alumina, while it leaves un 
touched the phosphates of those bases. The undissolved residue was dried, ignited, 
fused with silica and carbonate of soda, and the product examined in the manner 
already described. The addition of silica is indispensable to the retention of the 
whole of the alumina in an insoluble condition. Phosphate of alumina is not de- 
composed by carbonate of soda by fusion, or only partially, and is besides soluble in 
an aqueous solution of that salt. 
The results of the examination may be thus briefly stated: — 
Dark grey vesicular lava from the Rhine, used at Cologne as a building-stone, 
being exceedingly strong and durable. — Enough phosphate of soda was extracted 
from 1000 grains of this substance to exhibit the yellow phosphate of silver, and the 
phosphate of magnesia and ammonia upon a large scale. The phosphoric acid might 
be said to be here very abundant, that is, comparatively speaking. No attempt was, 
however, made to estimate it quantitatively, as the operation is attended with great 
difficulty, and the result of doubtful value from the unavoidable errors of experiment 
bearing too large a proportion to the quantity of the substance. 
W hit e trachyte of the Drachenfels, near Bonn, on the Rhine. — This rock is appa- 
rently as rich in phosphoric acid as the preceding; nothing could be more distinct 
and satisfactory than the indications of the reagents. 
Dark red, spongy, scoriaceous lava from V esavius .- — This was tried in the same 
manner, and yielded abundance of phosphoric acid. 
Compact, dark green basalt, or toadstone, from Cavedale, Derbyshire . — This sub- 
stance was very tough, and difficult to powder. Enough phosphate of soda was, 
however, extracted from /50 grains of the rock to exhibit very unequivocally the 
characteristic tests described. 
Dark blackish-green, extremely strong basalt from the neighbourhood of Dudley, 
termed Rowley-ragg, gave a very similar result. Phosphoric acid is not so plentiful 
in these substances as in the lava, although its presence is easily rendered evident. 
An ancient porphyritic lava containing numerous crystals of hornblende, from 
F esuvius. — The phosphoric acid was here very distinct, but not so abundant as in 
the more recent lava. 
A specimen of tufa, or volcanic mud, also from Vesuvius, was found to contain 
phosphoric acid in notable quantity. 
