HYDROGEN. A natural element, one of the constituents of water which is always formed when hydrogen gas is burned in combination with atmospheric air, or with, oxygen gas. It is. known to us, in its simplest form, only in the state of gas, and is speedily fatal to animal life when it is breathed unmixed with atmospheric air. It is, however, a component of animal mat ters, and it forms a very essential part in the economy of vegetable substances, in which it is. always found. Thus sugar econtnins 6.90 per cent. of hydrogen; gum 6.93; beeswax, 12.672; wood of the oak, 5.69; wheat starch, 6.77; acetic acid (the acid of vinegar), 6.35 per cent. It is the lightest of all ponderable matter, 100 cubic inches weighing only 2.15 grains. No known degree of cold has been able to condense it to a liquid. It can not support combustion, but is combustible in conjunction with atmospheric air. It constitutes one-ninth of the weight or water. Most vegetable structures contain hydro
gen in the form of water, but the hydrogen .essential to this constitution can not exist in the form of water. That hydrogen gas exerts a con siderable influence upon the leaves of plants, was first noticed by Dr. Priestley. Sennebier found that plants which lose their green color in the dark, preserve it under those circumstances, if a small portion of hydrogen gas is present in the _atmosphere in which they are placed; and Dr. Ingenhous noticed that its presence, when they are growing in the light, renders their color of a deeper green. Humboldt has noticed that plants growing in the galleries of the coal mines, pre serve their green color, although vegetating in the dark, and that, in such situations, the atmosphere contains a proportion of hydrogen gas.