ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 313 



a general way how we may establish, to some extent, the date at which 

 each species of the two groups is separated from the magma. Data 

 leading to the determination of their relative age are afforded by their 

 inclusions. 



A crystal developed in a vitreous mass frequently incloses particles 

 of the medium in which it grows. In this way certain sections under 

 the Microscope appear penetrated with vitreous grains, imprisoned in 

 the interior of the crystals and frequently arranged along the zones of 

 successive growth. These inclusions prove that the crystals in question 

 were formed in a vitreous mass, liquefied by heat. In other cases the 

 inclusions are mineral species in the form of microlites ; and it is clear 

 that they must have been anterior in date to the mineral in which they 

 are inclosed. Finally, in other (;ases, a species will mould itself around 

 sharply defined crystals, conforming to other outlines, and filling up all 

 the spaces between the minerals, thus showing that the crystals are of 

 earlier origin than the surrounding mineral. 



From these facts, which speak for themselves, we have been able to 

 draw up chronological lists indicating the relative date of crystallization 

 of each species of the two great periods. I will not stop to cite these 

 lists, but we shall soon see how the law which governs the successive 

 formation of the crystals, and their relative age, is evolved from 

 synthetic experiments. 



I have traced in broad outline the history of a lava, but have sketched 

 only a few of the details which modern researches on lithological phe- 

 nomena have developed with such startling reality ; lievertheless, what 

 we have seen is sufficient to show in a striking manner the power of 

 analysis when supported by reasoning. I think I am not wrong in 

 saying that from this point of view the study of a lava presents one of 

 the finest examples of the application of the inductive method to the 

 natural sciences. We hardly know whether to admire most the 

 analytical processes, or the subtilty of observation, or the logical 

 method by which the observed phenomena have been brought into 

 connection. 



Microscopic analysis, powerful as a method of investigation, has 

 enabled us to trace, with close exactitude, the progress of crystallization 

 in a rock where the unaided eye could discover only an indistinct and 

 uniform mass; to penetrate into this marvellous tissue of volcanic 

 products, where millions of polyhedra occur within the volume of a 

 cubic centimetre ; to determine, with mathematical precision, the nature 

 of each of these infinitively small bodies ; to track them to their birth, 

 and follow them throughout their development, tracing all the modifi- 

 cations to which they have been subjected under the influence of 

 physical and chemical agents." 



" The great improvements in the construction of apparatus, and the 

 application of the Microscope to lithology, have at length enabled us to 

 successfully attempt the reproduction of all the modern volcanic rocks." 



Microscopical Examination of Urine for Bacteria.* — Dr. von 

 Sehlen recommends the addition of boracic acid to urine, as it does not 

 precipitate the albumen, and acts as an antiseptic, thus preserving the 

 urine and its sediment for future examination. The solution is made 

 by dissolving 8 per cent, borax in hot water, then adding 12 per cent. 



* Centralbl, f. Bakteriol. n. Parasitenk., iv. (1888) pp. 687-9, 722-4. 

 1889. Y 



