Hahnium
Ha or Hn A name formerly suggested for element- 105 (dubnium) and also for element-108 (hassium).

Hammer mills
devices which are used in the chemical industry for crushing nd grinding solid materials at high speeds to a specified size. The impact between the particles, grinding plates, and grinding hammers pulverizes the particles. Hammer mills can be used for a greater variety of soft material than other types of grinding equipment.

Holmium
A soft malleable silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoid. It has few applications.
Symbols: Ho; m.p. 14740C; b.p. 26950C; r.d. 8.80; p.n. 67; r.a.m. 164.93.

Heavy water(deuterium oxide)
Water in which hydrogen atoms, 1H, are replaced by the heavier isotope deuterium, 2H. It is a colourless liquid, which forms hexagonal crystals on freezing. Its physical properties differ from those of ‘normal’ water; r.d. 1.105; m.p. 380C; b p 101.40C.

Helium
It is a colour less monoatomic gas. It is the first member of the noble gases (group  of the periodic table).
Helium is having the electronic configuration Is2 and consists of a nucleus of two protons and two neutrons (equivalent toAn a-particle) with two extra nuclear electrons. It has an extremely high ionization potential and is completely resistant to chemical attack of any sort