Dr Davy on the Use of the Microscope. 41 



gredient is in largest proportion. The same decision, from 

 the like inspection, may be confidently made on some of the 

 sands at a distance from the coast, which consist almost en- 

 tirely of siliceous matter, without any traces of comminuted 

 shells ; and, in each instance, chemical examination is found 

 to be confirmatory of the microscopical. 



I shall pass now to some instances of a different kind. 

 The unaided vision, as it is well known, cannot distinguish, 

 with sufficient exactness, precipitates of different substances 

 by their appearance, when they are colourless, as those of 

 the different earths, whether thrown down in the state of 

 hydrate, or in combination with an acid. But, calling in the 

 aid of the microscope, the distinction can often be made 

 without difficulty ; and, if not at once, it can be made with 

 little trouble by the addition of an acid, of one, namely, the 

 combinations of which Vith different earths, under the micro- 

 scope, have appearances that are characteristic of them. 



Alumina and phosphate of lime, — the one an almost uni- 

 versal ingredient of soils, the other a rare ingredient, ex- 

 cepting in an exceedingly minute quantity, — are, as is well 

 known, both precipitated from solution in acids by ammonia. 

 To determine which constitutes the precipitate, so important 

 in relation to agricultural purposes, is a difficult matter, 

 employing merely chemical means as tests, unless in a well 

 appointed laboratory. But with the aid of the microscope 

 the difficulty vanishes. Seen with it, the precipitate of 

 alumina, owing to its excessive subtlety, and the low re- 

 fractive power of its particles, has a hyaloid appearance, 

 while the other has a very fine granular aspect. The addi- 

 tion of a drop of dilute sulphuric acid will make the appear- 

 ances even more distinct. If the precipitate is entirely 

 aluminous, it will have a glacial appearance, and, on evapora- 

 tion to dryness, this appearance will be retained, the crystal- 

 lizations formed being not unlike those of ice in their gene- 

 ral character; whilst, if the precipitate is of phosphate of 

 lime, immediately on admixture with the acid, if properly 

 proportioned, needle crystals will appear in increased quantity 

 mixed with some delicate rhomboidal plates. And, in the 

 instances of mixed precipitates by ammonia, of alumina and 



