During some chemical reactions, luminescence may also occur. A well-known example is an oxidation of luminol, in which an oxygen atom is temporarily put into an excited state. The reaction of luminol is not fluorescence but chemoluminescence.

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In fluorescence microscopy, fluorescent proteins (e.g. Green Fluorescent Protein (GFP)) are used in, among other things, DNA research. This technique is called FISH: fluorescence-in-situ hybridization. Fluorescent proteins occur naturally in some species of jellyfish (Aequorea victoria) and coral (Discosoma). Thanks to genetic modification it is possible to give fluorescent properties to other animals as well. The best known example of this is the night pearl (fluorescent aquarium fish) which has been sold in Taiwan since 2003.

Fluorescence must be distinguished from phosphorescence, the phenomenon that a substance remains after being exposed in the dark for some time. This is used, among other things, in paint on the hands of clocks.

Phosphorescence is a special case of luminescence. It is the phenomenon that a fabric continues to glow in the dark for some time after being exposed. Phosphorescence can also occur when such a material is bombarded with accelerated electrons, such as in a cathode ray tube.

The term means approximately lights like phosphorus. White phosphorus does indeed give light in the dark, but with this substance this is caused by oxidation reactions of the phosphorus with oxygen from the air (it can also ignite spontaneously), and so the light has a different origin. In the usual naluminous substances, it is a result of the slow fall back of electrons excited by irradiation with light. The fact that this happens slowly is because the return of the electrons to the ground state in quantum mechanics is a forbidden transition.

While studying phosphorescence, Henri Becquerel discovered the phenomenon of radio-activity in 1896.

Phosphorescent paint was used in the past[source?] on, among other things, the hands and dials of watches so that they could still be read in the dark for a few hours. It is also used in light switches, so that they can be found in the dark, and in toys for a children’s room.

Phosphorescence should not be confused with fluorescence. Fluorescence does not involve a forbidden transition when the electrons fall back, so it will emit its light in a much shorter period of time.

Porosity or porosity is the presence of small openings (pores) in a material. One recognizes the word “pore” (small opening) in it. Porosity generally means that the material slowly allows moisture to pass through. It is also possible to suck in or suck up moisture by capillary action.

To insulate a building, a porous material is required, due to the heat-containing property of the large amount of air in the material.

Porosity can also help prevent metals from corroding further. These metals are aluminum, zinc, tin, chromium, nickel and lead. The metals corrode a thin layer. The corrosion layers are not porous, and the corrosion stops.

Porosity in soil science
In soil science the term porosity refers to the amount of pores in the soil. This is measured as a percentage of ‘inactivity’ in relation to the solid matter. For example, sandstone with a porosity of 35 % consists of 65 % sandstone and 35 % of other materials, e.g. gas or liquid.

In soil with good porosity, oxygen, water and nutrients can penetrate well into the plant roots. Too much water in the soil adversely affects the porosity. The pores are full and prevent good aeration, the minerals dissolve and can rinse themselves. Heavy machinery can also jam a substrate and adversely affect the porosity.

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A triklial crystal system is a certain type of crystal system that has a much more irregular structure than the other crystal systems. Characteristic of this type of crystal system is that no single crystal plane has the same dimensions (a ≠ b ≠ c) and that none of the crystal planes is perpendicular to the others. So, the coordinate system consists of three axes (a, b and c) of unequal length and the angles at which the axes are different each time (α ≠ β ≠ γ ≠ 90°). It is therefore quite difficult to determine at first sight whether a crystal is tricklial or not. To prove this, X-ray diffraction is usually used.

Despite their irregular construction, trikliene crystals are common in nature. Although the crystal system shows little symmetry, the crystal planes are sometimes of equal size. The most important property of symmetry in crystals is not the planes, but the angles between them.

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