Preparation of one or more subsamples from a sample of material that is to be analyzed chemically.

1. Reaction of hydrogen with another substance.

2. Chemical reaction in which an element gains an electron (has a decrease in positive valence).

Any process by which data are condensed, such as changing the encoding to eliminate redundancy, extracting significant details from the data and eliminating the rest, or choosing every second or third out of the totality of available points.

The lowering of a land surface by erosion.

The process of substituting for an observed value one derived therefrom, as the calculation of the corresponding meridian altitude from an observation of a celestial body near the meridian, or the derivation from a celestial observation of the information needed for establishing a line of position.