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POPULAR SCIENCE REVIEW. 
the No. 2 eye-piece, magnifying 350 diameters, the bubbles are well de- 
veloped, and appear to occur in most of the cells. Sometimes the cells are 
exceedingly irregular in form, sending out angular projections in various 
directions. The bubbles seem generally to occupy a large proportion of the 
cells, about one-third or one-fourth of the entire space, from which it might 
be inferred that the vapour from the condensation of which the bubble has 
been formed was not originally highly rarefied. Sometimes the cells occur 
in long slightly curved lines. Tubes and trichites are rare in the silica of 
this slice. "With regard to the granites of Co. Wicklow, he says the 
investigations of Dr. Haughton leave little to be added in the way of 
analysis. Nevertheless, the microscope reveals the presence of a second 
variety of felspar besides the orthoclase, which he and Professor Galbraith 
were able alone to detect from chemical examination. This second mineral, 
indeed, might probably have remained unobserved, except for the introduc- 
tion of this new process of investigation. The rock from which the thin 
slices are taken is finely crystalline granular, and consists of a siliceous paste, 
enclosing white felspar, silvery-grey and black mica. With polarised light 
the felspar is seen to consist of orthoclase and a triclinic felspar, less abun- 
dant but well defined. He says that the silica occurs in an amorphous state, 
enclosing the other minerals, as is usual in true granites ; but is itself com- 
posed of numerous individual patches, each refracting the light differently 
so as to represent, on rotating the analyser, a great variety of rich colours. 
The individual patches are generally separated by a fringe of prismatic hues, 
probably due to varying thicknesses of the section at the edges. With the 
L-inch object-glass numerous cells of varying shapes and sizes come into 
view, together with some wonderfully long “ trichites ” (“ trichiten,” Zirkel), 
some straight, others bent or curved, and stretching through the mass 
in various directions. They are quite distinct from the tubes, and I have no 
good idea regarding their nature. The cells are often disposed in lines, or 
along planes, perhaps concealed cleavage-planes. The cells for the most 
part contain fluid bubbles, but they are only visible with a very high power ; 
some, however, are “ stone-cavities.” Along with the cells and stone-cavities 
are also to be observed, with a high power, very remarkable straight tubes 
terminated by rounded ends, and evidently hollow or filled with gas. One 
of these, remarkable for its length, seems to contain a very minute bubble 
near the centre, and another at the end of the tube itself. 
How to Examine the Growth of Plants with the Microscope . — A method 
for this purpose has been lately described in an American journal. The 
inventor employs a glass tube of convenient size, to be placed in the field of 
a microscope, and allows the root, or other part of the plant, to grow in this. 
Of course the part must be fixed at some point, either with cork or with 
damp bibulous paper. The free end of the root has now room for growth, 
either in water or in moist air — preferably the latter. The tube can be 
subjected to a known degree of heat by the use of Sach’s hot-air chamber. 
The tube, having been fixed on the stage, can be accurately observed every 
few minutes, or after a longer time, a micrometer being all that is needed 
for determining distances. The errors which may result from these obser- 
vations are frankly alluded to. This simple method is particularly adapted 
to the investigation of the effect of light on growth, as the whole apparatus 
is completely under control of the observer. 
