Phosphoric Acid in Rocks of Igneous Origin, 297 



value, from the unavoidable errors of experiment bearing too large a 

 proportion to the quantity of the substance. 



White trachyte of the Drachenfels, near Bonn, on the Rhine.— ^ 

 This rock is apparently as rich in phosphoric acid as the preceding ; 

 nothing could be more distinct and satisfactory than the indications 

 of the re-agents. 



Dark red, spongy y scoriaceous lava from Vesuvius. — This was 

 tried in the same manner, and yielded abundance of phosphoric acid. 

 Compact dark green basalt, or toadstone, from Cavedale, Derby- 

 shire. — This substance was very tough, and difficult to powder. 

 Enough of phosphate of soda was, however, extracted from 750 grains 

 of the rock, to exhibit very unequivocally the characteristic tests de- 

 scribed. 



Dark blackish-green, extremely strong basalt from the neighbour- 

 hood of Dudley, termed Rowley-rag g., 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 phosphyritic lava, contairiing numerous crystals of 

 hornblende from Vesuvius. — This 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. 



These were all the substances tried ; they were taken, as is at 

 once seen, indiscriminately from igneous formations of many localities 

 and many ages, and they all, with one doubtful exception, in which 

 practical difficulties interfered with the inquiry, yielded phosphoric 

 acid. It is highly probable, therefore, that this substance is a very 

 usual, although small, component of volcanic rocks. 



It is not unlikely that the remarkable fertility possessed by soils, 

 derived from the decomposition of some varieties of lava, may be, in 

 part at least, due to the presence of this phosphate in the original 

 rock, although much must of course be ascribed to the alkali, espe- 

 cially potash, which these substances contain, and which is gradually 

 brought by the continued process of disintegration into a soluble 

 state. There can be little doubt that the matter erupted from time 

 to time from the interior of the earth, in a state of fusion, is thus 

 destined to renew the surface from which the more valuable and 

 more soluble components have gradually been removed by the action 

 of water and other causes constantly in operation. If it should 

 hereafter bo found, on a more extended investigation, that phosphoric 

 acid, although present in all igneous rocks, is most abundant in those 

 of modern date, the fact will thus receive an explanation, the more 

 ancient lavas having been most changed by the slowly-acting and 

 almost imperceptible causes in question. One might be attempted 

 to consider lava as a kind of fundamental material, from the subse- 

 quent alteration of which all others are derived, and expect it to 

 contain, here and there at least, traces of all the elementary bodies 



